Reverse lights / nuts size

[Ultrasummer]

Hey anyone have any idea what size the nuts are on the backs of the reverse lights in a 1968 stepside? The old one's were shot. Nuts are long gone. Nice new shiny ones are here now but no nuts. Went to the hardware store with one of the lights. They tried everything but could not find a nut that easily threaded onto the back of the new reverse lights. I looked around and couldn't find a thread. The one diagram called them "J" nuts. Any ideas?