Jim B PEI: You are 1 year off. Seat belts weren't strictly required until 1968, but the GSA* said for the 1966 model year that they would not buy cars unless they had seat belts, outside mirrors, back up lights, and a few other items. US automakers decided if these items were required to sell to the Federal government, they would make them standard. Seat belts had been offered as accessories on various US models since the Darrin and Federal vehicles had been bought with seat belts or had them installed after the purchase since 1962 or 63.
*GSA - Government Services Administration - a unit of the US government that handled purchases, leasing, and temporary assets for other federal agencies.