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Carsten,

Thank you for the description of your doublet (re)building / alignment method. If it comes to restoring the bond line of one or more elements, I'll have something to go on.

In the mean time, I have done a non-destructive experiment on the 1st reverter doublet from my telescope. This particular doublet appears to be degraded on one exterior surface only. Being plano-convex, it can be misleading which surface(s) is at fault. I wetted the far side (as I was viewing it) plano surface (the one I thought was degraded) with IPA and the transmission immediately improved. As the alcohol flashed off, the defect pattern again became visible as the last of the solvent film evaporated. Solvent applied to the other (presumed good) side did not produce any transient improvement and the defect pattern remained visible..

Further web searching found references to evaporated calcium fluoride lens coatings by J. D. Strong at University of California in 1936. Another mention was to Eastman Kodak's HECTA lens coating process in 1938. I haven't found a break down of the acronym, but it may be a MgF2 "hard AR" coating.  So there is some evidence that evaporated AR films were practiced in the US before my sextant's build date of 1941.