1920's era tub/shower valve set up with tub drain control

I need help on identifying the manufacture of a valve. There are three handles with the middle handle controling the tub drain. The tub spout is in the overflow spot on the bathtub. Another set of handles above for the shower. A parts breakdown would be nice. I know it is old and parts are rare, but the customer does want to disturb the original wall surround. If I could find a way to repair it would be great. The problem is the handle which controls the tub drain will not allow the drain to stay open. Once the handle is let go the drain closes.

I'm guessing the tub spout is also built into the side of the tub instead of being above it...
The spout is where the overflow plate and lever for the drain would be located on todays tubs...

If it is there is "0 Chance" of parts being available and all you can do is open the wall on the opposite side and see whats going on.
This is an indirect waste and the knob lifts a tube that is inside of another tube to open the drain.

Once you see it you may come up with a solution. Or, post pics.

There were several manufacturers of that type of system, but they all went out of business long ago, since it was before the depression. The drain has to go "over center" to stay open so something must be limiting the handles rotation. I am guessing that the spout is NOT at the overflow location, but instead is much lower, which makes it difficult to modernize those tubs with a conventional waste and overflow using a modern tub faucet. Those tub/valve combinations do not meet the "anit contamination" requirements of modern codes.