This was predated by the French Ideal which had the chopper-lump with vertical dovetail soldered construction since I believe at least 1900. The construction is well detailed in the 1910 catalog.
There are two types of dovetails here. The traditional dovetailed lump (one lump dovetailed on 2 barrels, everything being brazed together), and chopper lumps barrels with integral lumps, dovetailed vertically together, so that the barrels cannot separate because of longitudinal forces.
BSA certainly did it, but I believe that the "Manufacture" did it first.
They did say in their ads that the soldering allowed for less tube distortion during the assembly, because of the lower temperature used.
Best regards,
WC-