The labeling did not start until the 80's or so with the Bendix MV2 and later the MV3 valve.
http://www.bendixvrc.com/itemDisplay.asp?documentID=5108 If an earlier one is labeled like the MV3, someone retrofitted it.
From the early 60's until the Bendix MV2 which was superseded by the MV3, there was no standard for marking. If there is a red or yellow button, it was retrofitted Sealco had a button about the size of a silver dollar. Bendix had a shiny button about the size of a quarter. But they both worked the same. The button is spring leaded. To release the brakes, you push in the button and it release the spring brakes. Whenever air pressure drops to around 60 psi, the button pops out and the spring brakes set., To manually set the spring brakes for parking, you merely pull the button. This button was completely separate from the tractor protection valve. The tractor protection valve was usually operated by a toggle lever which merely supplied air our shut it off. The same lever is still used for suspension dumps and 5th wheel sliders. The tractor protection valve itself was usually towards the back of the tractor and was a spring loaded spool valve. When it received air from the toggle switch, it charged the emergency line to the trailer and allowed the service line to the trailer to pass air to the trailer service brakes. In the closed position, it allowed one to bobtail the tractor. It also closed in the event of a sudden air loss, it shut off the air to the trailer and therefor was called a tractor protection valve.
The old style valves are still readily available. The MV3 valve was a big improvement, but the old style worked. Much of this brake stuff was retrofitted. MGM invented the spring brake in 1956. It was a cylinder with a piston with an O ring and was very unreliable, but it was better than what we had before. Spring brakes did not become mandatory on trailers until those that were built in 1975. By then Anchorlok had invented the twin diaphragm spring brake that we still use today.
2Ton, I don't think the parking brake was ever a lever as you describe. It is probably a trolly valve that was designed to supply additional air to the trailer brakes. It may even be inscribed "not for parking". If it is some way hooked up to your spring brakes, someone probably jerry rigged it.
