I have an Open D rear frame looking for a home.
Also on the subject of seats. There were at least 2 different seat bases. The early one was made of interlocking steel 1 inch bands , bent to 90 degrees on either side of the seat and pop riveted to a flimsy pressed steel base. there was very little padding.
The second, that I know of, is a pressed steel base with a thicker padding. This one had a 1/4 bsf nut welded inside the seat at the rearmost corners to bolt the swan neck clips to. The earlier seat had the kitchen cupboard door lock at the rear and as previously stated, you could buy packing pieces to make the hinge line up properly.
Photos available if required. Paul.