Join Date: Oct 2005
I don't have my books with me and I might be slightly off on the details, but the story behind Monk's notches was that he was a Sherriff (bouncer) in a wild bar in his younger days. Every time he whacked somebody and threw them out of the bar he put a notch on his club.
When he reached 39 marks he clubbed a random person to make it an even 40.
edit to add,
Just looked at the wiki link it was for a even fifty.