[DonVoigt]

1. How common is this bolt failure? This is the only one I have seen like this.
NOT COMMON, i'VE SEEN PICTS OF SEVERAL AND HAVE ONE SLIGHTLY DIFFERENT(CAME IN A LOT OF USED PARTS"
COULD EVEN BE THAT SOMEONE TRIED TO ASSEMBLE WRONG AND USED A HAMMER TO FORCE THE EXTRACTOR DOWN INTO ITS SLOT, INSTEAD OF PUTTING IT IN FROM THE FRONT.

2. What do you all think the cause is? Poor metallurgy? Poor hardening? Excessive pressure from
hot rounds? Milling dimensions too tight on the extractor cut out? Or something else?
LIKELY WEAR FROM LOTS OF USE, BUT COULD BE ANY OF THE THINGS YOU MENTION.

3. If you have seen this in other Lugers, I am curious as to which ones? ie just WWII?, just Mauser?, WWI?, Commercials?
YES, OTHERS. USUALLY THE MAKER IS NOT SHOWN, SOMETIMES THE PIECE STILL WORKS- IT IS NOT NECESSARILY A FAILURE THAT WILL STOP THE FUNCTION OF THE LUGER.

4. Also curious about dents on back of trigger on both sides. Anyone seen that kind of damage. Is it from dropping the trigger when disassembled?
IT WOULD BE DIFFICULT TO DENT THE BACK OF THE TRIGGER BY DROPPING IT, IMO. MAYBE SOMEONE PRIED THE BACK TRYING TO REMOVE IT??

And a question for you-
Why are your pictures "greyed out"?