Yes, in or after. The rule and therewith the pattern of stampings changed in 1955.

As Alan points out, these are commercial; whoever bought these from the government had the responsibility to get them to one of the proofhouses and get them proofed. Birmingham used a helpful viewmark that could tell us the year, but London did not until much later.

And in case the OP wants the proofs explained: .45 is obviously the caliber, .900 the proof load case length, and 7 tons per square inch the pressure.