"Notch in your gun"

I am a reference librarian and was asked to help someone find the origin of the phrase, "Put another notch in your belt". I can't find anything on it in dictionaries, encyclopedias, or on the Internet.

Any ideas?

Thanks!

Notch in your belt sounds like something you would suggest to a successful dieter. Notch in your gun is more like it ... talks about the practice in the old west of putting a notch with your bowie knife in the stock of your gun for every man you killed.
Very successful lotharios have been known to put a notch on the bedpost for every conquest, as well.