Like these? I think they were seals on the seatbelt bolts that went through the floorboards. These are from my 74 Road Runner.
It is a rubbery material and there are voids (air bubbles) in the face that was against the floorboard. I doubt it was something applied wet on the floorboards - more likely they were made on some other substrate then peeled off and applied to the floorboards before the bolts went in.

Upon further consideration the color on the topside is gold overspray from when the car was painted. These were definitely in place well before the bolts went into the holes. Could have been applied wet to the floorboards, although that would be awful time consuming on an assembly line.