1972 A Body (Demon) Spare Tire Hardware/Install

It should be a straight Carriage bolt style one like shown up above. The cup is used in the fiber board but you are missing the small wing nut. Large wing nut goes under the fiberboard and holds the Jack base and tire down.
Nobody makes that cup so don't lose it. As for the small wing nut it is a stamped Style wing nut and may be available from a generic Hardware supplier