Both Ford and GM produced seat belt straps with four ribs in the 1960's and 70's. The 1969 Mustang had it. The mustang seat belt is 1 3/4" in width. Betting the the rest of the Ford belts were, too. As well as GM. Widely available in 1978.
I am guessing seat belts went to 5 ribs for better safety design sometime in the 70's and 4 rib type is probably no longer currently manufactured anywhere. I could easily be wrong about that. I don't know for sure but seems likely. There is plenty on ebay motors, though, in the form of vintage seat belts, or you can dig it out of a junk yard, which might be what the makers of the Fett costume did. Dunno that either.