While roll searching yesterday I found a penny that was otherwise normal except for the mintmark. My camera has no macro mode on it (surprising how it was one of the more expensive ones back in the day..) and the best shot I could even get with it wouldn't show in detail the little mintmark I'm talking about. When I first glanced at the penny I thought it was a 1963S, but when I pulled out my 30x loupe to look at it it looked like an S over a D or something. It's very strange. What it actually looks like is a flattened D in back with a small D tilted at a 45' angle in the very top right. This is a quick drawing I did in Photoshop about what it looks like: I really apologize for not having any actual pictures of the mintmark, but I know my digital camera can't focus on anything less than four inches away sadly. I can hardly get a detailed picture of a coin, let alone the small itsy-bitsy mintmark. Maybe there's a photo on the web or that someone here has with a coin with the same issue.
Tough to say for sure without a picture, but looking at your illustration, it seems like you have a RPM (Repunched Mint Mark). Prior to the 1980's (not sure of the exact year), mint marks where punched into dies by hand. As such, there are many reasons why you can find multiple mint marks on the same coin. Check out the varieties as catalogued by CONECA at http://hermes.csd.net/~coneca/. Hopefully, this gets you started. I'm not much of a variety collector myself, but there are plenty of folks here that are and can provide you with more details.
