There are holes pre-punched in the seat frame for them. They have a slight dimple around them, presumably to feel them through the vinyl. They're not real easy to find.
If they were removed holes in the vinyl would be obvious (if original material were still there) and the holes would have thread marks.

In reference to the color of the buckle head- many years ago I worked on one of the very early production run of promo red/red 70 Challenger convertibles; it had red C15 buckle heads. Quite possibly an early thing that was changed for regular production.

The other cool thing I found on this car was a small spot welded on sheet metal reinforcement piece under the hood latch tray. It appeared they found a weakness in the area where the secondary latch connects. The normal production latch tray has a revised design with more strength in that area.
I've seen this unique reinforced tray in one other very early car. Interesting stuff!