Early cars fitted for retractable seat belts?

As some of you may recall, my 70E Targa has been pretty extensively "disassembled" recently -- I just noticed that in the rear quarter areas there is a depression and a welded-in threaded fitting that appears to be factory provisions for retractable seatbelts. The odd thing is that AFAIK they didn't actually use the inertia reel belts until 73 or 74, right? I know my car has the original, somewhat decomposed, and annoying non-retractable belts.

Anyways, I thought this was interesting. Here is a pic of what I'm talking about. This is the driver's side, right below the targa bar area, with the interior panels and padding stripped out.