Hi Jordan,
obviously I cannot explain any historical events in your cars life but we can discuss possible scenarios.
For that you need to look at your data card first.
Was the car ordered with seat belts, for example, 431 or 434? If no option codes then I would deduce that your belts were installed in the USA.
That your belts have a 1966 tag doesn't mean much, imo, because it does not mean that it is a manufacturing date, it could be an approval date, or even that they were "NOS" Hickok belts when they were installed several years later.
My main point in all of these nauseating seat belt discussions is that Hickok belts were never installed or sold by the factory, they were strictly USA aftermarket belts.
This situation is quite similar to ordering a radio with a W113 car. If one was ordered when the car order was placed then the factory installed a Becker radio (or perhaps a Telefunken in other models like a Ponton). If it was not ordered, the dealership could still install one, either a Becker radio or any of the other aftermarket radios, but most often a Blaupunkt. So, is a Blaupunkt an "original" radio for a W113 car, my answer is "no", although it is certainly an appropriate and period correct installation, either by the Mercedes dealer or the Bosch dealer, or another independent shop.