FWIW, prior to 1968, many American gun companies, and those exporting to the U.S., had serial numbers based on the model. Colt, a worst case example, had one serial range for the Police Positive .38, another for the PP .32 and a third for the PP .22. Target versions had different ranges. The Officers Model had its own range, but the .22 version had a different range, etc., etc.. And of course, each auto pistol had its own range.
S&W was a bit more consistent, with each frame size having its own range, but didn't put any model identification or number on their guns.
So, police trying to trace crime guns went nuts, and even today both collectors and the folks at Colt trying to get information on a Colt by serial number often end up just plain confused.
GCA '68 gave the then-ATTD the authority to impose some system whereby the only information needed to single out a specific gun would be the maker and the serial number. Some makers (e.g., Ruger) used serial number prefixes to identify a model. S&W ultimately divorced its serial number entirely from the model number.
So, if a specific gun was made in that era, as changes were being made, and before widespread computerization, it is entirely possible that mistakes were made in record keeping, or even that two guns of different types could have the same serial number.
Jim