I have the same 2 threaded holes on my new chassis which I thought were for a seat belt, but yours were before seat belts came in?

I have two threaded holes in my 1967 AK350. They were hidden by the narrow (about 20mm?) flange where the front of the van cargo step sits on the chassis. (So, from the factory, the chassis had these holed but the body covered them). I drilled two corresponding holes in the flange in order to use them for mounting seatbelt stalks, although their diameter is a little smaller than the universal seat belt mounting size.
.......or were you talking about the two M7 threaded holes that later vehicle have to mount the central seat runner?