I have fully restored 2 sets of ND instruments this year myself 1000 / 750. There is no oil dampening inside them, simply a magnetic coupling
from the cable gear drive to the needle shaft drive and a hair spring on the needle drive base to bring it back to zero. So they can be stored any way you like, there is no oil in them. My 750 instruments would fog up all the time, Marcel, from cb750faces.com assured me that once I had restored them and put in new seals and LED globes the problem would go away, as the heat from the standard globes heated up the moist air inside the instruments and it would condense on the cooler glass. Well that's exactly what was happening, problem solved, no sign of any condensation now. No need to drill holes in the body either as the temperature inside them is now the same as outside using LED's. Was thinking about your tacho cable venting heat into the tacho body. If I thought that was the problem, I would get a small glass jar and drill a hole in the lid to put the cable through. First put some oil down the cable, put it in the jar with the lid on and run the engine up and see if any oil comes out of the cable into the jar. I'm thinking it would come out if there was any crankcase pressure in the cable, that could be a source of heat if it does. ?? Just noticed you are in Essex and i'm guessing its cold there now, so if any heat was coming out of your tacho cable then it would condense the air onto the inside of the cold glass jar, the same as it would do on your tacho glass.