1971 GMC Step side unknown plug below tailgate

[J01012814]

Hey everyone, I have a 1971 GMC 1Ton step side and have been doing front to back wiring replacement with an AAW kit. The truck has never had license plate lights as long as I've owned it and my grandpa who gifted it to me seems to remember it never having one either (owned it since the early 80s) I saw this odd looking plug on the rear just below the tailgate and I think at one point in time it was the plate light but I'm just not sure. Does have one wire running to it from underneath the truck but the original wiring is such a mess it's hard to tell what it is. Just wondering what type of plate light bracket it would've had and wanted to share a pretty odd truck. It's a 1 ton step side with the 9ft bed, 4 speed.