We find those occasionally too. I believe that the Ford part number is C6OZ-6362016-A - Spacer - front seat track to floor.
We list them as seat risers or extensions. For a boost to your bucket or bench seat.
If this is a common 1969-70 Mustang convertible installation part, I have a theory. We would have to make some comparison measurements of the convertible seat risers to prove it.
Ford called them 'upper floor cross members'. Basic Ford part numbers 10672/73. 1969-70 Cougar and Mustang did not share the same ones. Cougar platforms are shorter, due to the Cougar seats having more foam padding.
I postulate that Ford may have used a 'common', shorter seat mounting cross member, in all 1969-70 Cougar and Mustang convertibles.
Comparing the Ford and Mercury MPC's, gives me the same Ford numbers for those cross members. Both with Mustang part numbers. C9ZZ-7610672-A and C9ZZ-7610673-A. Coupe or hardtop models are C9ZZ-6510672-A and C9ZZ-6510672-A, which I know is wrong for a hardtop Cougar. The Dynacorn Mustang floor pans are about 1 inch taller than a stock Cougar hard top model. My guess is Ford did not support a Cougar part number for service replacement parts? (FYI fastbacks used C9ZZ-6310672-A and C9ZZ-6310672-A).