[51504bat]

I believe the seat is the same as a p/u. The p/u has a riser that takes the place of the under seat tank found on the big trucks. The wood bottom of the bottom seat has 3 indented holes that align with a stud on the riser/tank that allows for forward/back adjustment of the seat. The photo you posted shows the stud on your tank. BUT all of this is based on what I recall from the '40 1 1/2 ton conventionals I had years ago as well as a '41 1/2 ton cab. AND at 69 memory isn't my strong suit.