I've always wanted to find something like this on a 1980 D. Couse when you say 80 D. It sounds like A.D.D. lol. Cool coin. I think it's a die chip as well.
I don't think it's a RPMM. There are none that I see listed in lincentresources. It appears to be a tiny die chip. Nice find!!
It looks like a cool die chip. CopperCoins shows four rpm's of this date with split serif; the upside down pic of the mint mark above appears to show a split on the upper serif or it might be damage.