Looks like an ex-Forestry Service rig judging by the colors, and a 2-seater judging by the floor.
My guess is that some kind of recessed cup was there for a handle of sorts. I don't recall anything similar on my '70 Satellite wagon, and my '63 & '64 2-seat wagons of course had the spare well in the floor.
Otherwise it's about the right size for a courtesy light, which wouldn't really make sense.