Belated welcome to CT. Do get a Red Book. Even one a couple of years old is just full of information. About your mint mark. The mint marks were added by hand at that time, and as they were added, the punch could have gotten dull so the P isn't as distinct as it could be. Also in striking the coins, the low spot where the P is on the die could have picked up some debris/grease making it unclear.
It's just a little chip inside it. You want to always ask, "Who the hell would want to collect this, anyway?"