Heisz wooden bar clamp

[olderdan]

It looks to me as if the clamping block is too square on the rail to jamb in position, it needs to be able to tilt more. You could screw a thin plate on the rear to engage in the threaded rod, if all else fails I would drill a series of holes along the length of the rail and just use a pin through as in a sash cramp. Seems a shame to lose all that nice work.