I will say that is a plating issue. It can't be a RPM (Re-Punched Mint Mark) because in 1993 the Mint marks were already incorporated onto the Obverse Die. They were no longer punched by hand onto the Die. It is either a small Die Chip or a plating bubble.
That's a neat find and a definite keeper. Probably a plating error or a neat die chip. It could be a combination of both. Nice find!