Did the screw drop out the lock? If not can you turn the lock with a screwdriver? Counter clockwise I believe. This is the only pic I have found: https://www.ipdb.org/showpic.pl?id=2564&picno=67684&zoom=1

That bar needs to be pulled down to release. If the lock is drilled all the way through can you thread a stiff wire with a hook on the end through there and catch that bar and pull it down?

Could you grasp the end of the bar with a long hemostat and give it a twist I wonder? You may have to drill the lock out more to make a hole big enough.

Impossible to try with the head still mounted but I wonder if gravity would pull those arms down if you could lay the head on its back and gently drop it a couple of inches