. . . Do you mind if I ask what reference you used for the April 1951 date? I may want to use it for another S&W gun I have . . .
I use my own data base, compiled as I find verified information from various places. To the best of my knowledge there is no readily available SN reference source for most M&Ps, other than the serial number range by year in SCSW.
Many of us create such a data base for particular models of interest. DWalt has an excellent one for several models . . . including yours . . . and others will chime in when we can add to the available information.
Put us to the test by posting your other example. I would suggest starting a new thread for it so it doesn't get lost.
Russ