This is what you are looking for. Here is a 1939 Registered Magnum showing the Registration number stamped in the frame of the gun. Each one is custom ordered and has a mark just like this.
If you are a collector there are 3 basic types.
1) Those that are Registered with the Factory and a certificate issued.
2) Those that are Registered but a certificate was not issued
3) Those that are not Registered but have a Registered stamped frame
You could probably subdivide it even more, but the real problem comes down to the factory records. If when you letter the gun, the factory historian can find the paperwork on your gun, or you have the certificate, then you go into #1. If not but it is a low number, say below 5000, then you go into #2. If still not, and you are a high number, then your letter says you have a "registered model", meaning they cannot be sure. Mine shown above is one of those. The difference between #1 and #3 is probably in the range of a few thousand dollars for the same gun.
If I see another low serial number and I can afford it. I will pick it off quickly! I was bummed when it turned out mine was a #3 gun.
Great shooter though. Truly amazing gun!