Generally, trade makers usually did assign their own serial numbers to guns they built for others, but some did not (Wilkes comes to mind). The maker's number was usually stamped somewhere near, or on, the fore-end loop and, of course, this number usually won't match the retailer's number. By the same token, some retailers (such as Wilkinson, later Manton's, etc.) didn't assign their own numbers, and the guns were sold with only the maker's number on them. Generally though, a gun built for a retailer will have a non-matching trade maker's number on it.

While I'm not certain of the earlier guns, those that Webley & Scott built for others after the merger were stamped with their own number. If the number is absent, or is not correct for the time frame, then it came from someone else.