I made several questions to the audience, after noticing that Steven D. Hughes in "Doubles Guns" 2007 stated that amongst side-by-sides, there was a 7 pin Holland & Holland Type Sidelock (pp.65-66), with a characteristic pin below the tumbler axle. Most gunmakers in Britain and the Continent copied it afterwards or produced derivatives of it. The other important type is the Beesley-Purdey-Atkin family of spring openers, with 7 pins but with a different arrangement over the plate, also recognizable at first sight. Now, from time to time other external pin arrangements have come to my attention, i.e. a S. Grant with 8 pins (a Twelve-Twenty), a Beesley and an Atkin with 6 pins, and so forth. (Aside, of course, of side-by-side guns of the 5 pin type).
On the other hand I have found another common 7 pin arrangement in most Hussey guns and also in some Lang and Grant shotguns, clearly different from the H&H pattern.
All of this without discussing the pin arrangements of Over and Under sidelocks.


HOLLAND & HOLLAND 7 pin pattern.
https://photos.google.com/photo/AF1QipO5gv78XJiZD54O_MlEixK-bFERYM1n9ECLrr8H?hl=es

HUSSEY (also in some Lang and Grant) 7 pin pattern.
https://photos.google.com/search/_tra_/photo/AF1QipOkBrkFts-hGMTQ6uyI_m_Bt048ycFSlaYHtFY4?hl=es

Note: Shotgunlover has a point, lock makers may had nothing to say in the design, it seems so far that gunmakers had favorite patterns. Only a statistical analysis or archival evidence could say the contrary, as to whether the Wolverhampton lock makers had their preferred types.