Jam a big screwdriver into the lock and see if you can turn it. And the bar inside with it.

LTG : )

Or increase the size of your bit

You still have to turn the thing that was locked. Drilling just means it's not locked any more.

You NEED to do the LARGE blade screwdriver first then a large drill bit if that doesn't work, then back to the screwdriver again just as suggested.

Just to articulate what everyone is saying a little better, there is a long bar that runs across the top of the head. It swings forwards like a lap-bar on a rollercoaster and holds the glass “down”. What you want to do, and what keys do, is turn that upwards, so it is no longer keeping the glass down in place.

The glass probably moves up in the corner because the bar is no longer anchored down by the lock. There is still a pivot point on the other side of the head that the bar is attached to. The only way to free the glass is to use a large, flat screwdriver to twist the bar up.

Read the previous post:

If you still can’t figure it out get a cheap inspection camera you can stick in there and see what you need to do to free the glass. https://www.harborfreight.com/digital-inspection-camera-61839.html

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.

Edit.

Whoops - I initially posted the only picture of a backbox from the GTB 1982 parts catalog, but the lock position does not match your game so I deleted it. BTW, the 1986 Premier catalog (which includes Super Orbit) shows no backboxes at all.