Thanks, I seem to have a liking for these budget bikes, this one is for sale I might get it as another project. I like the head badge.Yes the Triumph frame is bigger. There is a difference in the ages of the two models. Your BSA is an earlier model with steel bars , stem and cranks. There is also a variation in the seat stay ends which are fluted. It seems as though they were fashionable at some time .
I think they are basically the same frame though .