This information is not in any particular order and is waiting for me to sift through it and expand on parts of it as I learn more.
After the first World War many nations were investigating the possibilities of producing a practical Aeronautical sextant. In 1924 Mr. Booth of the Royal Aircraft Establishment, produced his bubble sextant and it was adopted and became known as the "Booth" or more formally, the RAF Mk V sextant. It was a successful design and much used by air navigators. At around the same time the Americans introduced a very similar instrument which became known as the Bureau of Standards Model. This would form the basis of many of the later American designs. The Mk V was further developed, becoming the Mk VIII and became the standard RAF bubble sextant. However with the improvements in the design and performance of aircraft, better instruments were needed and in the thirties Mr. P.F. Everitt, chief designer of the long established instrument makers Henry Hughes and Son, produced a new form of bubble sextant using some elements of the earlier types. This instrument was much easier and quicker to use and more suited to mass production than the RAF Mk VIII (also made by Hughes) that it was to replace. It was given the nomenclature Mk IX unsurprisingly, and was introduced into the RAF just in time for hostilities. Later versions of the MK IX incorporated clockwork averagers, single and dual speed, and later still, a simple star telescope. The model sequences were Mk IX, Mk IX A, Mk IX B, Mk IX AM and lastly the Mk IX BM. The last model was used in the RAF up till around 1970 in non pressurised aircraft. The RAF Mk 2 periscopic sextant for use in pressurised aircraft like the V Bombers, betrays its origin with the same clockwork mechanism and similar ten degree increment setting. The RAF Mk 2, also made by Hughes, is, on examination, very advanced for its year with a pendulous "horizon" and the T model having a polorised filter for use in the polar regions. Interestingly, there was a MK XII sextant produced by Hughes, I have seen photographs of it so it did exist, but I have never seen any mention of it in the usual literature.
1938. RAF took into service the Mk IX Sextant.
The original instruments had no glass in the eye piece, later a modified plain glass lense was added.
At some point the bubble light filter wheel was changed from a plain 6 hole disk to one with a mixture of plain and red filtered holes. The original filters were made from gelatine and will immediately shrink and become slimey if they come in contact with water. I, as a matter of course replace mine with modern plastic material.
Early users of the instrument were reporting poor results being obtained, there was a rumour going around that the instruments used on tests were superior to those issued. The RAF chief Navigator and, I believe, Francis Chichester, studied normal serving navigators using the sextant in real flying conditions and realised that Navigators were defeating the purpose of the manual averager by only actuating it when they "thought" the sight was good. It was decided to add an "Automatic" device to remove actuation from the operator. This is where the A model comes in.
A complaint from aircrew was that the Index mirror operating knob was far too small to be used with gloved hands. Aircraft were flying at much higher altitudes which meant permanent wearing of gloves was mandatory. I have a modified Mk IX where a hand made knob of much larger size has been fitted.
Photo of Modified Mk IX
It was also realised that the rather complicated tilting prism arrangement for bubble illumination was not required. A simple frosted glass replaced this and in fact is superior in operation.
Photo of Illumination Mod.
The British were very interested in German equipment and they subjected all captured material to scientific investigation and evaluation.
Here are the RAE and Admiralty reports on captured German sextants.
You will have noticed that the internal parts of the sextant are exposed to the elements because of the large opening used to give the mirrors visibility. Whilst I have never seen one, the RAF literature states that the instrument should be stowed within it's box inside a drawstring bag. Where these bags came from I don't know as I have many instruments arrive to me full of protective packing wood shavings. These would not have been able to enter had the instrument been in a bag.