John,

The seat was widened in 1950 by 2-1/2" and in 1951 the improved tracks and adjustment rod instead of the cable were introduced. This system remained in use through the 1955 1st Series.
http://www.flickr.com/photos/advance-design_parts_co/8251910066/in/set-72157632186114015
http://www.flickr.com/photos/advance-design_parts_co/12315510004/in/set-72157640522200463