Joe,

I'm pretty sure that the impression you mention is from the attach equipment. There's something unusual about those small brownish steel snaps: They've shown up primarily on 3-6 sheaths! Here are some examples:

The two I own:




Two others that have shown up on e-bay recently:







All of the associated knives have circa 1960 characteristics. The only other sheath I can remember seeing with these snaps is a 2-5:



So the question is why do we see these almost exclusively on 3-6 sheaths and not a random mix? Another "Randall mystery" to add to the collection!