I've read the website from the US mint about it, but I'm still confused.
http://www.usmint.gov/about_the_mint/co ... mint_marks
I read:
Proof coin operations were moved from the Philadelphia Mint to San Francisco in 1968, and the letter "S" identifies that special numismatic coinage.
Does that make a penny special just because it has "S" on it? I have several of them from the 70s.
Also, I have some from the 70s/early 80s that have no mint mark at all. What does that mean?
Does a simple mint mark alone make a penny worth more, or is there more to it than that?