They are called Cinch Fasteners. Originally available as single, double and triple length. seem to have been maily used by Ford, 1932 to 1947 according to one source.
One online catalog Restoration Specialties, see page 177 of their 2023 catalog
https://www.restorationspecialties.c...log_ep_43.html
Mac's auto used to have the double as UF2 and single as UF3. Ecklers took over, only seem to have UF2
https://www.ecklers.com/search?keywo...ehicleType=All and have gined a reputation for "streamlining" inventory and being near the top of the price range for many products...
Another claimed seller
https://www.oneillvintageford.co.uk/...x.html#SID=220 They say 4 in stock, hope this doesn't mean this is another part that the original/repro supply is drying up as seems to have happened to the fasteners along the bottom of the side curtains on Cab 11/12.
See also
https://www.classictruck.ca/interior...-10-32-machine