Steve, I think it was the later cars that had a plastic clip down on the inner sill which was indeed to hold the seatbelt catch, to kind of keep it tidy when not in use. Your pre-prod car may not have had these?
The production DHCs had an L shaped metal guide mounted as part of the seatbelt assembly for the seat belt to run through. These weren't needed on FHC cars as the belts ran through a guide in the trim. I think (although not 100% sure) that these DHC guides were handed left & right to slightly angle the seat belt towards the wearer.
I am sure one of the specialists could help you with these parts.
Tony
<font size="1">1981 2L FHC Cavalry Blue
1980 3.5L V8 DHC Jaguar Regency Red - sadly sold!</font id="size1">