Steam Heaters under compartment seats

Does anyone have any information on how the steam heaters located under Mk1 compartment seats work, we are currently dismantling the ones out of our BSK for refurbishment. We have found a fair amount of debris inside them. Inside the finned tube there is a brass inner tube with a steal rod inside that which has disintegrated, there is an adjustment screw and locknut and one end and somehow this works as a thermostat which regulates the flow of steam into the chamber but we don't have a drawing or info to see if any parts have wasted and are missing ect. The open style coaches with longitudinal heating pipes have an aluminium tube inside which works on thermal expansion with a ball bearing and spring which prevents steam entering when the require temperature is reached. We have made one of these from scratch and tested it and it works a treat, so the plan is to manufacture enough of them for the open coaches. However its the Compartment heaters which are slightly more confusing as to how they work.,

If anyone could give some guidance on how they operate or drawing that would be much appreciated?

Cheers

Dan

EVR