Isnt that plug the same as used on 70 mustang taillight buckets and nj package trays among other uses as they had a hollow hole in them to screw over pointed screw end to protect sharp point. These were hard to find till i found a bunch of them at a mopar swap meet back in the 90s as they were used for a weatherstrip screw protector but here are 2 types a hard plastic and a softer rubber type.