Hi Buckstix,
Unlike in France, I have no evidence that the British trade worked that way. Adams and Tait (A&T) made a lot of barrels for the trade, but I don't think that they assembled them, most likely provided them in sets L/R. Most probably without the ribs, jointing, chambering, etc.
I don't know what this number means, except that W Richards might have bought that gun in the white from another maker.
This was standard practice. Both Webley and Scott did most of their business that way. Scott was making the vast majority of H&H guns until they had their own factory at the very end of the 19th. Webley made guns for most everyone. Even Purdey subed some guns to others including Henry.
Fraser himself had his custom actions built by Webley.
Best regards,
WC-