Mandatory Seat belts?

[chtr65]

Can anyone shed some light on when Chevrolet started putting seat belts in Suburbans and trucks? I know 1966 had Chevrolet seat belts. My 64 Suburban has aftermarket seat belts that have the nubby belt material. I have been talking with someone very knowledgable about the Corvettes, and passenger cars. He tells me that the 64 should have had Factory seat belts, and a webbing material as apposed to the nubby material. My thought is that the 64 Suburban had owner installed aftermarket seat belts. Any one have solid info to back this up either way? Thanks in advance