Nothing with Gibson was absolute at any period. In the fifties the FON(serial number) was stamped in the rim. From the sixties on it was stamped on the back of the peg head. This was the usual design, there could have been exceptions at any time.
I think the only rims stamped with FONs in fifties were the very earliest years, maybe some rims up to ‘54. After that, back up on the obverse of the peghead up until, if I am correct, the early 80’s, when the reissue models started once again, stamping the rims.
I’m not positive on this, only going on personal observations, and the usual disclaimer, anything with Gibson was possible.
According to banjophiles some of the banjos were stamped in the rim as late as 1959. Don't assume that this was absolute practice. Some RB250s were still made with arch top rings after the flathead ring was reintroduced and some were made with rosewood fingerboards after ebony boards were introduced.