OK guys.. I just wanted to state that it looks to me that it's not the letter M. I studied Greek and that is actually the symbol for SIGMA.. There is a difference if you look carefully.. Oh gosh here I go nitpicking again!
I have never done anything with it, I found it in a box of junk about 10 years ago, saw it was dated 1812, and put it in a display case. I just happened to be going through my cases last week, and thought to find a forum to see if anyone had an opinion. On the back side of the coin, is a deep spot of the coin missing. I am guessing this is probably environmental deterioration?
203 years have passed since your coin was minted. I think it's just wear. It must of went through so many hands, pockets, coin purses etc many times.
If a severe cud or die crack was found, the first thing they would probably do is retire the die before it shatters. Scratches and (simple) die cracks would probably be ignored. I don't believe they look for doubled dies. A strikethrough is a one-time occurrence. One of my favorite books about the development of the U.S. Mint is From Mine to Mint (1833-1937) by Roger Burdette.
Perhaps that is how I eventually acquired mine, a waffled MO SQ. I sold it on eBay a couple of years ago. Chris