sure someone will be along to tell you for sure but on the few pre war Bsa underlevers that I have seen, the piston washer has been held on by what appears to be a countersunk head 2ba screw. I have come across two examples where this has broken flush with the piston head and I have centre popped it, drilled very carefully and then tapped to 2ba. (This takes some doing without a vice and with a drill stuck on high speed!)