Battlewagon3

Hello. I'm working on a short blurb about an older car with advanced safety features for its day, and I'm trying to determine when backseat seatbelts were required in the United States. I'm not talking about "seatbelt laws" requiring people to wear them, but the requirement that they be included on new cars as OEM. I know that they were required in the front seat from 1968 (according to AAA's website, although I thought it was longer ago), but I know that by the 1970's most American cars (and foreign cars sold in the United States) had backseat seatbelts. Some of the information I've found online has been incomplete or arguably wrong, but the main problem seems to be that every web search turns up information on "seatbelt laws" rather than requirements for manufacturers. Does anyone know when backseat seltbelts became required OEM in the United States? Thanks.