I noticed when I saw the lead photo in the auction catalog (see below) that this was not just another Brandis nautical sextant, it was a Byrd sextant. Byrd sextants were one of the first types of sextants specifically devised for aerial navigation. The leveling device was a spirit tube mounted to the frame beyond the horizon glass. It was the spirit level, and a wire leading from it to the sextant’s handle (the spirit level could be illuminated at night by a small light bulb that was powered by battery in the sextant’s handle), that led me to recognize what I was looking at. A Byrd sextant was aboard the NC-4, the U.S. Navy Curtiss seaplane that made the first-ever flight across the Atlantic Ocean in 1919. But by the mid-1920s, the Byrd sextant had been replaced by other types of bubble sextants.
I know of only a few other surviving examples of Byrd sextants, all of them built on Brandis nautical sextant frames. One example, Brandis #5296/USNO #2977, is at the Smithsonian Museum in Washington, D.C. ; this Byrd sextant flew on one of the Navy-Curtis seaplanes involved in the NC-4 transatlantic flight of 1919. The Smithsonian’s web site says that this particular sextant was donated by P. V. H. Weems, the U. S. Navy officer who was one of the great pioneers of early air navigation methods and equipment . A second Byrd sextant, Brandis #5292/USNO #2975, also flew on one of the NC seaplanes that attempted the 1919 transatlantic flight; it appeared on an auction web site several years ago and presumably is now in a private collection. A third Byrd sextant, Brandis #4180/USNO #2859, belongs to Bill Morris, author of the excellent book The Nautical Sextant , and the equally excellent blog that complements the book .
Those are the only three surviving Byrd sextant I know of; quite possibly there are others, but I suspect not many others. Just from the standpoint of its rarity, I thought this Byrd sextant was worth having. But when I looked at the additional photos in the catalog listing, this sextant seemed even more interesting. It has a micrometer scale rather than the tangent screw and vernier scale that is seen on the other three Byrd sextants; the clamping mechanism is also different (see below). So I think this may be a somewhat later version of a Byrd sextant than the other three, the micrometer and clamp features intended to make it easier to use in flight.
One photo in the auction catalog shows that the sextant’s box has a Naval Observatory inspection certificate attached to it (see below). The original Brandis serial number written on the certificate, 5996, is crossed out and written below it is the number 6006; photos of the frame of this sextant don’t show us which, if either, of these two numbers corresponds to its Brandis serial number. More interesting is that the inspection certificate indicates the class of sextant to be “aeronautical”. The inspection certificate for Brandis #5292, one of the Byrd sextants from the 1919 transatlantic flights, indicates it to be a “surveying sextant”, as do the inspection certificates of many U.S. Navy sextants that were used strictly for nautical purposes. I think this lends support to the notion that this sextant was from the outset made for aeronautical navigation, not for maritime navigation. Another interesting thing is that in the upper right hand corner of the inspection certificate is written “Weems 805W”. I don't know what the “805W” refers to, but “Weems" must refer to the very PVH Weems whom I mentioned above. It seems possible that this particular Byrd sextant was once in the possession of the great air navigation pioneer himself during that early period when he and others were working to advance the design of the aeronautical sextant.
In summary, while the three other Byrd sextants appear to be retrofitted Brandis nautical sextants taken from the run of 2400 sextants that Brandis produced for the U.S. Navy during the first World War , the Brandis Byrd sextant I failed to win at auction seems to be a somewhat later sextant specifically made for aeronautical use. The Naval Observatory inspection certificate is dated October 8, 1920, so this provides a ‘no later than’ date for when Brandis #5996 was manufactured. It seems likely that it was made some time after the Byrd sextants that flew on the May, 1919 NC transatlantic flights, so the date of manufacture of Brandis #5996 was probably some time between May, 1919 and October, 1920 , so it fits nicely into the Brandis serial number chronology I’ve been developing.
Comments, corrections, additional relevant facts, differing viewpoints, etc., are always welcome (no one will be banned, blocked, or castigated for offering differing opinions). Send corespondance to firstname.lastname@example.org
Footnotes and references
 The Nautical Sextant, W. J. Morris. Paradise Cay Publications, Inc. Arcata, California. 2010.
 See the earlier post titled 'What do the Numbers 3500 and 1542 Tell Us? Part Three'
 The actual Brandis serial number of the sextant I bid on was more likely 6006 than 5996, but I think that date range likely applies to Brandis #6006 as well.