At first glance, it sort of appears to have a massive strikethrough error on the reverse. But I know that’s not really the case. It has to be PMD (post-mint damage), or, as @paddyman98 says, a “DEFDAM”. (Definitely damaged). But it is rather intriguing damage, nonetheless. I’m at a loss as to what might have caused that localized area of (corrosion?) in that bizarre ovoid pattern, almost in the shape of a capital “D”. Eh. Who knows. Weird, and mostly worthless, but interesting, huh? Maybe acid-etched, but if so, the pattern is still rather baffling.