This mint error is more commonly found, and most valuable when, the 1941 PR is missing the "AW" or "Monogram" or designer initials. The AW stand for Adolph Alexander Weinman whom designed this coin.

Certain Kennedy halves are also well known for this same type of error, though the 'FG' (for Frank Gasparro) is missing on those. From what I can tell, 1982 is the most common, but many years are affected.

That's goor info and all of the missing DI's on all coin series would make a decent article for coin collectors to read.