Fitting the record unit to another make, I have done it to a Mk3 webley, Obtained an HW35 with a ruined cylinder, as i didnt have threading facility on my lathe at the time I bored the hole in the front of the HW rear block bigger cut the threaded portion of the block off & grafted it onto the front of the webley block that had been turned down. It worked. A better way would be to mill the slot in the webley block bigger to accept the trigger unit.
Or you could Use BSA Airsporter bits, The threaded portion on the Airsporter & Mercury is steel & is bolted onto the front of the alloy rear block with a big hollow bolt that facilitates adjustment of where it locks up when you screw the block into the cylinder.
So easiest way to convert a Prewar BSA without skilled lathe threading Is to take an HW 35 rear block cut off the threaded bit that goes into the cylinder & bolt an airsporter cylinder end plug onto it. It should be a relatively easy drilling & threading job. The HW block will need to be turned down on the outside.
BSA Airsporter & Mercury share the same cylinder threads as their prewar ancestors so breech blocks interchange. Think the Webley Mk3 is the same size.