I stand corrected regarding rear seat belts in the 1962-63 cars.They were not factory installed but could be dealer or self installed. Attached is the 62-63 Jetfire assembly page for rear seat belts and how to add to a 63. I followed this procedure when I added them to my 62. I do not recall if the dimples were present in the floor pan or not?
I have also copied the seat belt listings from the 1962 and 1963 Parts and Accessories Catalogs. In 1962 they just list front and rear seat belts, no color options or separate hardware. For 1963 they list many color options and a Seat Belt Aux Pkg for the rear. I have assumed that this rear package is the hardware for installing whatever color seat belts you want.