I ran across this a type of hull plug that has the name Tri-Sure. I do not know if it is original but it sure looks of the period. I am trying to track its origin but has anyone seen something like this before?
No, they had nothing to do with GPAS. Your bung looks to be from a aviation fuel drum going by the inscriptions on it. The bungs and the gpa inspection holes were both NPT thread, National Pipe Thread,and the same diameter.