First year for standard shoulder belts?

[DANTIP]

I have a 1972 1/2 ton truck with lap belts and was ticketed in Indiana for not wearing a shoulder belt (my lap belt was on). The law reads that if shoulder belts were standard equipment and installed by the manufacturer, they must be worn. Optional shoulder belts for 67-72 trucks are very desirable and were definitely not standard equipment. I suspect the belts became standard in 73-75 but don't know how to prove it. Is there a document that proves the year the shoulder belts became standard?