SKB, it appears the serial numbers on the receiver and barrels were struck with different dies. Note the 1s and 3s for example. I wonder when making the gun if the same dies were used on both barrels and receiver. From what I "think" I see, it looks like the gun was rebarreled and then reproofed at a later date.

docs.google.com/document/d/1PqK3UImDCrsOrk6O_2DsovHXvaNSzliSiwaP35Y6PMk/edit?tab=t.0