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Fergus.  I might have misinformed you.  Did you mean a Mk1 Sextant Mount or a mount for a Mk1 Sextant.  Believe it or not, there’s a difference.  I’ve just checked the mount for my Mk2 Sextant.  It says it’s a Mk1C Mount for a Mk2 Sextant, so when they changed to the Mk2 Sextant, they didn’t change the name of the mount, they just gave it a different letter after Mk1.  There’s one on eBay at the moment described as Mounting Mk1F for Periscopic Sextant Mk2 from Hercules C3 XV221, so there were several slightly different MK1 Mountings.  Mine was from a Dominie.  So my revised answer is that it depends upon the letter after Mk1 in the mounting.  That's always assuming that the mounting for the Mk1 Sextant didn’t have a completely differently named mounting altogether.  DaveP

Fergus Kavanagh wrtote:  Question: Do you know if an RAF Mk2b will fit a Mk1 sextant mount?

In short, I think you’d need to modify the necks at the very least.  I have a Smiths Mk2 and what I believe to be the civil version of the RAF Mk1.  I also have a Mk2 mounting. The tube diameters are both 57mm within 0.1mm, and the grooves are the same width and the same distance apart.  However, the Mk1 tube is longer above the top groove, but shorter below the bottom groove.  Moreover, on the Mk2, the azimuth ring is on the sextant, but on the MK1 it must be on the mounting, because my Mk1 only has a lubber line, no azimuth ring.  The Mk1 will go into the Mk2 mounting past the first interlock, and you can open the hatch, but it won’t lock up, because the second interlock doesn’t make.  Surprisingly, although the Mk1 tube is slightly longer than a Mk2 tube, the head doesn’t stick out quite so far from a Mk2 mounting (see photos).  So to summarise, a Mk2 sextant might fit in a Mk1 mounting, but you’d probably have to remove the azimuth ring from the mounting first, and even then, it might not work.  Paul Brewer knows much more about working on these sextants than me.  If you have a Mk1 mounting, please supply some photos. DaveP