Don't know about Boss. I can't recall ever seeing a Boss double rifle.

I know for sure that Purdey uses their own in-house regulators.

Don't know how the numbers fall, but Purdey was certainly more prolific as a rifle maker before the nitros. I've wondered if that might have been due to the fact that their proprietary range of nitro DR rounds generally flopped? The quality of the hammerless nitros is almost always fantastic and thus highly desireable, and I agree that the lower numbers help keep the prices up.

Holland always promoted their theoretical edge in strength. Their hammerless sidelock DRs have always been backlocks, whereas Rigby and Purdey were barlocks. Purdey did use a bolstered frame on their nitros from early on, but Holland added a bolster in the 1920s as well. Rigby didn't use one until very late in the game. Ironically, it's the Hollands that seem to have the issue with staying on face.

Westley's droplock is an Anson & Deeley body action gun (boxlock) with hand detachable locks that can be removed through a hinged floor plate. The original purpose of the design was apparently to eliminate the visible ends of the pins that the lockwork of a boxlock are mounted on, as Leslie Taylor had decided they were unsightly. The primary benefit is precisely that, but the removable locks are easier to clean, or repair in the field - if you have a spare set of locks.