Front Bottom Seat Frame: 123 vs 126
 

I'd thought I'd read here that the bottom seat frames for the 123 and the 126 were interchangeable, and without really looking at them (I had a good 123 bottom to replace my 126 seat frame) I had it reupholstered and assembled.

Well, I noticed the the plastic trim piece covering the seat hinge seemed ill-fitting, but that didn't set off any bells. But when it was installed, the seat belt receiver didn't operate correctly. There was interference with seat bottom when moving the back of the seat up and down.

Luckily, I hadn't tossed the 126 seat bottom, and as I looked at it, there were cutouts in the sheet metal where the upholstery fastens to the outside of the frame. These are shown in both pictures. On the 123 seat frame that metal is continuous. Once I cut out the metal on the 123 seat frame, the trim fit correctly, and the seat belt receiver operated smoothly.

So I'd say that a 126 seat frame bottom would probably work in a 123, but you need to rework the frame a bit to swap them the other way.