More on threads.

We have a few new SMLE cocking pieces. I suspect they were made the same time as the SA barrels and bolt heads. These later bolt heads will only take a No4 Extractor screw (5BA) and likewise the contemporary cocking pieces have a 4BA (like the No4) thread for the striker retaining screw.

All the later editions of the SMLE component drawings have most of the threads changed to the BA system. So later produced components can be affected.

To quote Michael Caine, "Not many people know that". Better to be aware than wreck something.