There's a mechanism inside the backbox that has to be turned, which moves a bar away from the backglass. Normally, the lock would turn it.

I don't quite recall exactly how the mechanism is designed. You might have to get some sort of square bit and fit it in the plate that is supposed to turn, or just jam something in it, like needlenose pliers, then turn.

I try to avoid drilling backbox lock completely through so that there's enough left to try to turn it.

Or if you drilled all the way through try laying the head on the ground backglass up and the lock bar might fall down out of place. Then remove the backglass with the head still laying on the floor.

Closed and open positions. Late so probably worthless by now. But just got mine

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