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Great! I opened it, that went well. The mechanism didn't run as free as I would have hoped. Hours of searching resulted in the main shaft for the main mirror was well stuck. So some careful dismantling and cleaning helped there. A tiny drop of oil here and there in the process also made the mechanism smoother – I don't know if it it supposed to run dry as some shutter parts in cameras are. 

Then the smaller 5-degree mirror didn't move to well either, so I had a go at that also. The averager mechanism works but a puny spring is missing so if the sextant i turned upside-down then it will likely jam. Funny thing I see absolutely no remnants of the spring, it must have escaped the black dome somehow before. 

What definitely remains is the bubble. There is a drop of fluid in it but not more. I have ordered 1L of Xylene and will use that for the bubble in my Link A-12. Would work here as well or? So the next step is to attack the left side of the sexant. This is all very fun and my experience with some camera's minute mecahnics help. (jabcam.wordpress.com). I have taken lots of photos when reassembling so I hope to describe it on the camera blog.

Thanks for your detailed advice!