It’s speculation, but these SSZZA4 stamps started out perfectly crisp, but maybe they were not hardened properly they and started to deteriorate with use - the “earliest” suspected use of the stamp shows perfect symmetry and completeness. The stamps quickly begin to look like they had to take broken pieces and cobble them together to keep using them. Some of the last ones are missing the “bird” completely and only show the sszza4. You really have to ignore the sszza4 stamp somewhat and look at the rest of the evidence to accept the rifle, such as the serial fonts, components used, stamp location. It’s like anything, there are specific patterns to follow. I put much of this info in Volume 2 SS chapter.