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Attached is a sectional drawing of the bubble unit of a Soviet IMS3 sextant. 
The instrument has many similarities to the SOLD, as the latter was made by C 
Plath, whose satellite factories were over run by the Soviet Army towards the 
end of WWII. The bubble units appear, at least outwardly, to be identical. 

114 in the diagram appears to be the screw that, with washer 115, closes the 
filling hole. The fluid was probably xylene or hexane, rather than the 
alcohol solution or filtered kerosene used to fill compasses. Xylene (or 
hexane in the UK Mk IX series) was chosen because its refractive index is 
close to that of glass and so the upper "glass" of the unit would not act as 
negative lens when the chamber was filled.

The drawing has five pages of cyclostyled text in Russian referring to it. I 
can scan and send these to anyone to whom they might be of use or post them 
on my blog. Let me know what you would like.
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