I have made 4 or 5 of these pans. They are all one piece, as I remember, except for the mounting nuts to the front hinge. I used to make them with a slip roller. I made the roll for the partial cone first and then I hammered the surrounding area flat. The originals were all pressed steel, but I could not turn up the back end easily so I welded it on. It was upholstered so it did not show.
There was a subframe to the seat instead of a foam. The subframe was strips of spring steel crosswise on the subframe rails and on top of that was a horsehair pad, then the vinyl.
I have an original pan around that I kept to duplicate new seat pans. I am not exactly sure where it is, but I will take a look.
The subframe was damaged beyond repair, but I saw this photo of a subframe sitting on a tub. It is damaged on the front timing side, but you can see how it was. Sorry about the low resolution. With luck, someone else will have some photos.
View attachment 31723
Most owners do not use the subframe and replace it with foam. Shown in this photo are the rear spring clips to hold the seat down on the frame of the open models. The enclosed model uses a furniture catch to hold it to the tub.
I don't think yours is an original seat pan. The original pan has a round, or partial cone shape to form the clearance for the rear tire. Additionally, the original pan had holes stamped into the flat perimeter of under part of the pan. These holes had steel barbs stamped into them so the upholstery could be done without rivets.
David