Quoted from vid1900:Put a large, slot head, square shaft, screwdriver into the head keyhole.
Use an adjustable wrench to turn the shaft.
About 1/8" turn and the head will open.
Turn it 1/8" back, and you can lock it again for transport.
Bring one to every non-pinhead pin purchase, because the backbox key is always missing.
When I bought my Bad Cats, the back box and coin door keys were missing. So we just drilled the lock out.