Regularjon: Thanks! That helped a lot!
It seems what I'm after is a 4.5" Weston meter.. I'm only able to find the one with 3.5" diameter so far though, and not 4.5". Am I correct in that that 3.5" is a Weston model 301?
And, even if a meter is sealed, can't you just cut it open (To change the back plate) and weld it back together afterwards?
You also said 100uA, from what I understand it doesn't HAVE to be 1mA, you just have to make some resistor changes in the meter circuit to compensate for the different meter, yes? I found some info on it on the forum here so I'll read more about it.
EDIT: Did some more searching..
Look at this:
http://jlegerstee.trustpass.alibaba.com/product/234294573-0/Industrial_Meters.html
Here the scale says 0 - 1mA DC, so there should be possible to get the Hoyt Model 574 meters with specs that fit the GSSL compressor it seems. I can't for the life of me find a price or somewhere to easily buy it though ???
I'm perfectly fine buying a new Hoyt meter instead of an old weston meter, I just want it to be big and round.
I guess this thread should have been posted in the black market..?