Absalom
SWCA Member, Absent Comrade
My dad's revolver (pre model 10) that he used has two serial numbers; one from S&W, the other from NYPD, but there is nothing else saying "NYPD" or similar on it.
If that is your dad's service gun, the second number is not a serial number, but should be his shield (badge) number. Do you have his badge or any documents from your dad's service to check?