I’m writing up my notes-thoughts on RRC 290/6 for the paper on small change for the RBW conference next week. I just wrote this sentence:
The unciae looks like the type was designed to be used on a denarius that was never struck.
As I stare at the two known specimens I wonder if I see traces of the uncia denomination mark having been carved out of what was meant to be an X behind the head. Or maybe its just confirmation bias…
The third known specimen is no use as this portion of the die is off flan.
Liv Mariah Yarrow 
Confirmation bias, I think. Since the denomination pellet (or X for the denarius) would be incuse on the die, it wouldn’t be something you could “scrape off”. Instead, you’d have to fill it…a difficult process. I suspect just a die flaw (although maybe the celator engraving the head of Roma die was used to doing denarii and started the X, said “Oops” [or the Latin equivalent thereof] and stopped, putting the pellet above it…but that is just wild speculation on my part).
yes agreed