All About Pocket Knives

I have a tree brand boker 220 senator pen knife. Any one know how to date this knife?
I bought it from a co-worker, maybe early '80s. I have seen a few for sale, but nothing about when they were made. Thanks for any info.

If you go to this topic you will find alist of tang stamps.
http://www.allaboutpocketknives.com/kni ... =4&t=39799

Thanks shearer, that was a lot of interesting info. I am going to search the internet for info on a Joseph Rogers 4 blade congress I have. It is a used knife a friend gave me after high school in the late '60s.