What is this hole and cover for?

[eric67m]

I picked up this cab from CG. "Thanks again Mark" Neither of us could figure out what this is for. The cab is a 1970 very basic model with emissions. Behind the passengers side where the top of a fleetside bedside would hit there is a perfectly drilled/punched 1 7/16 hole. Covering over this hole is a 3 7/16 circle (not very well cut) held on by three sheetmetal screws. The cover is painted the precieved to be original paint color on the underside and was sealed down with black goo / silicone.

Does anyone know what this hole would be for? I know there is a bolt on fleetside beds in approximatly this location. Because of the metal window supports and emission tubes they would have had the bed removed to make this hole, did not make it from the inside.

What would mount here or this be an access for? (I know I'm probably opening myself up for all sorts of comments, so be nice)