What is the difference between the USP45 and the USP45F?

The "F" is usually used to denote a fulsize USP.  A "C" is used to denote the compact.  That way there's less confusion over which version of the USP you're talking about.  I don't think it's anything official, just sorta the way it's done.

For instance, we have no idea if the first USP45 you're talking about is a compact or fullsize.  We do know the second one you're talking about is a fullsize.

Ross

If you go by H&K's designation its pretty simple. USP 45 is fullsize. USP 45C is compact. No designation means fullsize, C means compact.