I would think that this type of bracket was added as an improvement to an earlier design. This one looks like it's designed to keep the seat from moving from side to side on the stands.

Remember there were two kinds of seats - the standard and the deluxe/extra padded seat. I have an Extra seat hanging in my garage rafters that has these brackets on both sides. It was out of Ryans (Blacksnake) 52. I believe the 53+ seats are wider. So, I can't help but wonder if this bracket was added later on (like 51/52) of for the Extra seats, and what you have is an earlier seat - say 48-50! Maybe your truck originally had a 51/52 seat with the brackets in it and someone replaced it with an earlier seat, and just tossed th ebracket where you found it?????!!!

Let me check the books and come back if I find anything.

 

Julie-Denny's pic is a '49 seat and has only one bracket. How can I tell the difference between a regular and deluxe seat? My truck is a deluxe cab so I would like to have the correct seat. I have access to a number of seats, so if I can determine which one is a deluxe I can pick it up when I go to get another bracket from one of them. I'm assuming that the seat isn't the original because the bracket was cut off the floor and the bolt stub is still stuck in the floor. Do you have arm rests?
Ross- the arm rests are repro's that I installed into the factory door screw holes. Dennis Carpenter #BAAA-8124141. I covered mine myself in vinyl to match the door panels, not an easy task but do-able.

 

The Deluxe seat had foam rubber top padding. The regular sets were burlap on the springs. They were the same seat except the deluxe seat had the extra foam rubber cushion ( about 2 inches), and was upholstered to accomodate (perhaps in two tone for Extra Interior)

Deluxe Seat covers were another option.