TYPE P4A Compass

I just acquired a TYPE P4A compass, serial number 17527 T. It has an Air Ministry tag REF Nº 6A/745. I’d like to restore it to its original condition. It seems intact and functional, but needs cosmetic refurbishment, which should not be a significant problem. The biggest issue is that it is dry. I don’t know if the fluid evaporated in the intervening 60 plus years, or if it was drained. It obviously needs fluid damping as it takes a quite a long time for the needle to settle down (very low friction pivot). I know a variety of damping fluids can be used including water, which would not be appropriate for an unheated aircraft at altitude. Since an aneroid vents into the bowl of the compass, it seems obvious that air (or some gas) must be left in the sealed bowl to allow for gas compression and expansion to equalize internal and external ambient pressure as the aircraft changes altitude. Does anyone have pertinent information about the fluid and the size of the air/gas bubble that was originally used? Did the Air Ministry publish a maintenance manual for the TYPE P4A compass? I’d be grateful for any help that can be provided.
Ray