Bolt Size on seat belt

[daddygo]

Need help with a bolt size on the top seat belt harness attachment in the Targa roll bar.
I have a ROW 1973 Targa that never had 3 point seat belts installed. The mounting nuts are in the roll bar frame. I thought they looked like 12 mm x 1.5 thread, but they (both sides) bind up after a couple of turns by hand. Looked inside the mounted nuts, and the threads look fine, but maybe after 45 years they have enough corrosion to bind them up.

Any suggestions?

[3rd_gear_Ted]

Those are SAE threads, all seat belts are SAE back then. Porsche didn't deviate.
Common tech question

[Trackrash]

The threads are either SAE 7/16s fine or 11mm by 1.25. Those two sizes are interchangeable. If using SAE use a grade 5 or higher, and metric 8.8 or higher.

[daddygo]

Thanks for the input.