They fit on the rear end of the Canon FD L 300mm f2.8, 400 f2.8L,500 f8 ,600 f4.5, 800f5.6, and 1200 f11, and the front end of many more of Canon FL series lenses and Canon range finder 7 series camera lenses.
darin,
A quick reference for finding the filter size on most any lens is the listings @ KEH Camera.
The filter ring size is identified with parenthesis '( )' for most lenses they list. Very handy!
Random KEH listing example for a Canon lens using 48mm filter:
Canon Manual Focus - Fixed Focal Length Lenses Dead Link Removed