I remember JT saying several years ago that there are some entries in the CPG in which it is difficult to establish just what may have happened but some have been included simply because they are interesting, and this is one of them............ CPG 4th Edition Volume 1, p.119 Though we will not say this is an RPM, we will say it exhibits what we like to call "character." Size and shape very much match that of a second D mintmark punch. Further study may prove it to be a die chip. Chris
Look closely at the OP's blow up on just the "5". You can easily see where the die chip starts about at the inside of the loop on the "5". If it were the mm punched there, the edge would be at the outside edge of the "5". Yes, I overlaid that too.