I'm not sure how GTB backboxes of 1984 were, but I encountered a similar issue on a Title Fight which was made several years later. That game had the lock & cam on the left side similar to yours (more toward the middle iirc). The lock cylinder connects to the bent end of a long flat rod stretching across the top of the backbox; its opposite end fastens to an internal pivot bracket with a screw. The rod's span has offset portions kind of like a crankshaft, to engage with tabs in the top of the box to lock the backglass in place.

Look at the cylinder you pulled out: see the threaded bore at the end? That's what the lock's latch/cam/bar/whatever attaches to: in this case, the end of the rod I mentioned above.

In my situation the owner lost/broke the keys and drilled the lock so it was already loose, but the glass would not lift: same as your problem. But I got lucky in that even though I didn't fully know what/how I was looking for, I somehow managed to get a screwdriver into the end of the rod that would have attached to the lock, and was able to turn it. I honestly had no idea what I was dealing with until I felt the mech turn and yield, was able to lift the glass, and then could see inside and it all made sense. It was easy to replace the lock.

So maybe your case is similar? With the lock cylinder removed, the end of the rod is free and should be dangling somewhere near. You'll have to engage and turn it, which may take as much luck as skill. The hole in the rod should be square, so your screwdriver needs to be wide enough to wedge in: use the biggest that will fit. If it won't free up turning one way, try the other: the lock mech enforced a specific direction and range so with that gone you might be dealing with more range that is not necessarily "correct". I don't have any pics available but hopefully this description helps.