I now have four MkIII's in my collection; two are marked Hughes and Son, the other two are marked Kelvin and Hughes. The sextants all differ in small aspects from each other. The most intersting is that the clamps only seem to work on the sextant they are mounted on indicating that they were specifically cut for one sextant only.

HO316 is the first survey sextant I ever bought. It originally came without the telescope but after buying another 'half' sextant with a telescope but without proper mirrors, two sextants became one. It originally belonged to the Hydrographic Office of the UK (UKHO) and has seen much use based on the wear on the index arm. The box I have with it is also a separate buy as it came without a box originally.

HS125 is a bit of an enigma as it reads 'Sextant Challenger Mk3' and is labeled HS rather than HO. There was a surveying ship called 'Challenger' in those days (1950s) which became famous by its last captain (later rear-admiral) Steve Ritchie; a well know person in the hydrographic world untill his dead in 2012. Besides the book, the Challenger became famous for measuring the (then) deepest point in the world, called the Challenger Deep. So if this is the sextant from the Challenger it has a great provenance. However, the Challenger was part of the 'regular' Hydrographic ships and a sextant would normally be labeled HO rather than HS. To make it more complicated, the book on sea surveying by Alan Ingham from 1975 has an image of this sextant labeling it the Challenger Mk II sextant. So it could well have been the model rather than a reference to the vessel. If anyone can shed any more light on this, feel free to inform me. Until then I will believe it is from the Challenger...

More information:

Sextantbook

Hydro International

Telegraph obituary