Hi arch, without seeing it up close, and knowing if the "mod" is intact........It's quite hard to know why it was done.......A normal thing to see is a metal collar brazed or pushed on the end, to enable corks to be fired at fairgrounds......I have read somewhere that the BSA 1930's "breakdown model".......if tested at the factory before dispatch, and found not to be up to the normal accuracy expected, was reserved as a "fairground" sale.......ie. was kept back to be sold to shooting galleries, as it was popular with these people...I also heard/read that the foresight on this (BSA) model is set back further from the muzzle to allow for the cork tube thingy (is there a proper name).......to be attached..
OR......this all be total twaddle, and I was dreaming....no doubt someone will be along to confirm/ridicule this this story as appropriate