The key says Lincoln Mercury. What was it used for?
looks like a spare tire lock.
sidemount tire lock
Still made today, have one on my spare trailer tire. Covers one of the lug nuts so no wrench can loosen it.
Takes a special lug nut. The special nut can be made, but I haven't had one in many years.
Brattons Model A makes those special nuts.
Place it on one of the spare lugs, when mounting the wheel. Then that accessory locking cover fits over, the lug of the key lock engages the grooved shank of the special nut.