1945 S Lincoln Wheat Penny.

That is a ball serif style mint mark. On these you look at the top loop and see if the thinnest area is at the top. They go very thin at 12:00 on the mint mark. Here is an example from coppercoins:
http://www.coppercoins.com/lincoln/...ie_state=mds
The ball serif style were the common one for this year. The rarer for this year was the new serif style. But most don't save them. Your coin appears to be normal, but appearing wider than normal. Looks the circulation wear has widened it some.