There are some shipping date oddities about the postwar .38 I-frames (Regulation Police and Terrier), I have two RPs with the expected four inch barrels, but they have the round butts associated with Terriers. One is numbered 54804, the other 58470. Both shipped in July 1951. Both are the flat-mainspring "unimproved" types. I think it is possible that your Terrier would have shipped about that time too.
These are neat little guns. Can you post some pics?