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Many thanks, Bill and Peter! I just realized I should have said more about why I am asking. I am not planning to build an ocular. Rather,  I managed to find both a Tamaya Coutinho-style sextant, and a Plath Coutinho sextant in its original circular metal case.Both have a funky Galilean scope with a vertical slit feature. I thus assume that this particular scope is an essential component of the AH system for the sextant, and my guess is that it is designed to minimize viewer parallax relative to the bubble. 

Both instruments will be added to our instrument collection at the Stanford Rumsey Map Center next month, so what I am trying to find is something that talks specifically about this type of scope.  In addition a friend who is an optics expert wants to read more in hopes of writing up something definitive about they particular type of scope.

Any leads, however incomplete would be gratefully received!

best

-p