I've seen three 99xxxx guns that have the serial number very heavily stamped. Two of them appear to have had their frames re-ground on the serial number side to remove the serial number (making the area of the frame at that point very thin) and then the serial number was restamped making it look like it was stamped very heavily. Not sure why the orig. serial number was ground off though...maybe the digits were badly mis-aligned North to South. One of these is pictured in JD's latest book...page 190 where you can see the heavy stamped number and a grinding ridge above the hammer screw.
We have a D98xxx range gun that also received the same treatment.
These heavily stamped sn guns were all shipped a few months later than they should have been, but that also happened to be when they changed the rollmark on the barrel, because the reworked guns do not have the SR in the eagle logo.
Chet15