The COBRA lettered belts are stock for CSX2201 up Cobras including all the works racers. They are extreme duty fully cloth wrapped. There are more than a few still in use. There are zero markings on any original belt I have looked at beyond the white ink printing. The white ink printing is real fragile. Any solvent or pressure washing (the used car lots of decades ago) would easily remove the lettering. SAI offered a service replacement for years after production of cars ended. More than one September 1966 dated service belt has been found.
Before work on cars besides Cobras (GT40s, GT350s, 427 Cobras for example) the parts numbering system used was very Fordish C4SA- prefix and based part number. The original part number was C4SA-8601 and a suffix that has so far been lost to time. The belts are marked
'COBRA SA-8601 MADE IN U.S.A.' plus a date code like 'K 66' for the September 1966 service part I have.
I have been providing art work for people to make their own stamps for quite a few years.