Today I've studied the coins carefully, in better lighting in the daylight. I think I know what the defect actually is now..

I think it was the top coin in the coin roll, where the machine that rolls/stamps the ends of the coin rolls most likely pressed into the end coins. I probably just ended up with the top coin out of a fresh roll, there's probably a matching tails side coin defect from the bottom end of the roll out there somewhere...

Probably not quite worth as much as I was hoping, but still neither have been circulated, so I still plan on keeping both of them.