Researchers will now be able to add to present knowledge, by comparing images and dimensions of the subject Oosterwijck with Coster’s known Hague clocks, also with comparable European pendulum clocks. I recommend the following pointers to horologists and researchers, as being probably rewarding new lines of enquiry.
 
Salomon Coster’s pendulums fall into the earliest phase of Hague clocks. His known pendulum timeline is from his June 16th 1657 Patent, assigned to him by Huygens for 21 years, to his decease in December 1660. Of course he had to be involved even earlier in Huygens’ initial trials and preparing Patent Applications.
 
In Coster’s pendulum oeuvre, time-pieces having ‘square’ pillars are regarded as the earliest of his few extant clocks. all being attributed to John Fromanteel under his Contract with Coster on September 3rd 1657. Clocks having ‘round’ pillars are thought to be later. As a consequence of this rather uncertain methods of dating, Dr.Plomp’s chronology puts Coster’s earliest extant timepiece D1 bearing scratched date 1657, among his post-Contract Oeuvre. Whereas, Huygens' earliest drawing of his 1657 weight regulator, (I assign it reference, 'DØ1W'), shows baluster pillars.

 

fig. OR1 (click to enlarge) 
The Box Cases of the 'Royal' and three Coster Hague Clocks.

Extant timepieces, alarums and striking clocks, attributed to Coster, all have four-wheel trains; all have plain unsigned movements; all have four pillars, all riveted to the front plate and pinned at the back plate. All his single and split-barrels are front wound. All have round dial-feet, except D5 (square), D8 (hexagonal). Timepieces have set-up ratchets mounted on front plates, his Alarum 'D5' has the ratchet on the backplate, whereas his striking clocks have internal set-up ratchets now on the front barrel cap (like Oosterwijck's). Coster’s ‘going’ trains all have the first wheel planted behind the barrel; centre pinions fix centre wheels; all have vertical trains to the 3-spoked contrate and verge-escape wheels; all have short verge-staffs held in Dutch block-potences; and naturally, all had Huygens’ patent crutched-verge with his suspended pendulum and cheeks (several are reconstructed).

Immediately one questions, why did Oosterwijck plant his centre-wheel at the front? And why did Pierre Saude too, in 1659, when Coster purportedly had set the only model used by the French makers? (see “Huygens Legacy”, Nr.14, pp.40-41).

Why the paucity of extant pendulums made by Coster himself? Coster? 'D3', found incomplete in France, by Gerd Wijnen, has an unconventional barrel cap, fitted into its barrel by a curious dovetail; it then had a tear-drop pierced, sculpted, hour hand of an earlier type. Berry Van Lieshout suggests this method of fixing the barrel cap might relate to Coster’s apprenticeship using Renaissance techniques. Privately, he suggests it could well be the only true ‘Coster-Coster’ pendulum extant. Yet its square pillars place it, too, within the Contract period, as that single feature is attributed solely to John Fromanteel. Less obvious autograph features are being ignored; we must identify and qualify all these too.

Fromanteel’s signed and dated 1658 timepiece looks like Coster’s. However, its six square pillars are riveted to a taller back plate, pinned at the front; its set-up ratchet is on the back plate; its watch-stop work is on the going barrel at the front cap, having a five-wheel train, planted vertically, with an intermediate wheel (for longer duration) fixed to its arbor by a pinion at the back plate (like Coster centre-wheels), with its centre wheel reversed to the front plate, to the now rebuilt Dutch potence block; having Huygens’ horizontal escapement and his suspended pendulum.

Bartram’s little clock c.1659, has four tapered square pillars, pinned at the back plate, now with two five wheel trains; the centre wheel at front plate; having two back-wound going-barrels in place of a split-barrel, each has diminutive watch-stop work set on the winding squares at the back plate; now with a pivoted pendulum (but signs of a Dutch escapement). His going wheels all have three spokes, typical of the Hague clocks.  [Bartram’s relationship with Fromanteel is presumed; Livery Company records show that in 1655 Simon Bartram, Thomas Loomes and John Fromanteel were all Sureties to Ahasuerus Fromanteel; this probably refers to the younger son, (apprenticed to Lionel Wyth on 21 June 1654, whom Bartram had taken over during his apprenticeship), who was made Freeman on 6th July 1663].
 
Edward East made several early spring-driven wall clocks with beautiful large dials, all-over florally engraved in his goldsmith trained free style, all having Fromanteel's pivoted pendulum, a single hand, some with alarum, and all having the strike on a second barrel. One movement was exhibited at Octrooi op de Tijd, nr.40, (Museum Boerhaave, 1979; reproduced in Early English Clocks, plates 104-107). Originally it would have had an ebony Tabernacle case, like his 'Huddleston' clock (Lloyd, H A, 'Old Clocks'. plate 15c). East also made a small box-cased timepiece, having early four-wheel train, now restored with verge across the plates to a pivoted pendulum in the manner of Fromanteel. Its centre wheel is at the front plate, its ratchets too. The signed backblate inscribed with a spurious date 1763. East's bold baluster pillars are unlike other London or Hague pendulum clocks, but are seen in Bernard van Stryp's contemporary Antwerp clock. East's going-barrel extends in four protruding lugs, with holes by which the front cap is pinned. It is unconventional, but also seen on the anonymous nodding-Chronos posted clock, (formerly Ilbert’s, now in the British Museum), which -on other grounds- I have likened to Davis Mell’s musical automaton chamber clock, I have attributed both those  clocks to Ahasuerus Fromanteel Senior.
 
Are these Fromanteel, Bartram and East pendulum movements just the distant English cousins, or the natural brothers, of the Royal Oosterwijck?
 
What is certain but is usually overlooked, in 1657 and later, any English pendulum construction that mirrored the Dutch spring clock format, whether made by the Fromanteels, Bartram or East, had to be derived directly from an actual Coster clock in hand, i.e., not from Huygens’ intentionally diverting "OP" drawing of a half-seconds weight clock that he published first in “Horologium”, (September 1658). Huygens' original seconds' weight clock, presented in Coster's June 1657 Patent, was not published until "Horologium Oscillatorium" in 1673.
 
Severijn Oosterwijck's Royal clock proves that, during 1657-8, he was close to Salomon Coster and John Fromanteel then in Coster’s employ, as well as having Huygens' confidence. The fact that his clock bears his own signature indicates he nevertheless remained independent and had his own clientelle. We also know, in May 1660, Alexander Bruce (Earl of Kincardine) joined Charles II in The Hague for His triumphant return into England. It is likely that Huygens' new pendulum clocks figured in their discourses, leading to their contacts with Huygens and Oosterwijck, also arousing Bruce's subsequent interest in the pendulum's, supposed, Longitude applications, which he independently was to pursue on his return to London; very probably using the Fromanteels to develop his "F" forked crutch which he first showed to Huygens in London during 1661.
 
However, we do not know when Severijn first had contact with the Fromanteels' workshop in London. Did he, like John Hilderson and the Roussels, make that short sea crossing somewhat earlier? If he did not, then similarities between his clock and Fromanteel’s early practices are even more remarkable. However, if he did have earlier contact, that might resolve the secret of the September 3rd 1657 Contract, between young John Fromanteel and prosperous Salomon Coster – who curiously pledged his entire, present and future, wealth to meet its arcane terms.
A catalogue raisonne´ of Severijn Oosterwijck’s Oeuvre would be invaluable. Several are identified by Dr.Plomp, Mr.Vehmeyer, and “Huygens’ Legacy". The “Lieberge”  silver-mounted timepiece-alarum is another of Severijn’s early gems, ascribed to 1658-1662, it sets new heights in precious ostentation.

 
Fig. 3a1 (click to enlarge) 
Oosterwijck's exquiste, silver-mounted, 'Lieberge' Hague clock. (Images courtesy of Sothebys Amsterdam).
 

Lieberge's cheek cocks are like the subject clock, but its reversed verge-cock resembles Coster's. It is an early example of a Hague clock being signed on the backplate, and it introduces his trademark plate outlines. (Sotheby’s Amsterdam, 21-02-1995, Lot.324). Two later Oosterwijck clocks, one also signed on the backplate, are depicted by Dr.Plomp, ("Pendulums", Op.Cit. nrs.84, 85).