Designing Kitchens and Bathrooms - Stubborn Drain

The tub and drain in my home are from the 1930's (I think). The drain is controlled by a knob on the wall. The drain no longer stays open unless the knob is held. When I let go of the knob it clunks back to the closed position. The drain itself is a plunger style. The knob on the wall controls the plunger. It looks like gravity is pulling on the plunger which pulls on the knob. Does anyone know what keeps the drain open?
Note: This is not a common lever style drain.

1900-1920--turn knob to right it locks--turn to left it opens. or just the opposite, I can't remember.
Finding parts may be a problem...............
We had one on first house-never broke.......
Check w/ local supply plumbing shops-not big box-
Cross Plbg. in Detroit propably has parts--313-836-2110

Haven't seen one of those in years but I think they basically work with a notch and knob type mechanism. The drain itself is "normally closed" by gravity. At this point the knob is not engaged into the notch. When the handle is rotated the knob finds the notch and holds the mechanism in the open position.

Over time both the notch and the knob wear out. If you can get in there it may be repairable but I doubt it. Most of those things were cast from "pot-metal", the same stuff the old tin soldiers were made of around WWI & WWII. :)

Have to revise dates fron 1900-1930.
Found our original abstract-house built in 1924.
good luck on finding parts.............
PS: wish I could have taken house w/ us when we left Detriot......................