NavList:

Fergus, I'll try and answer as I go through your post.  Paul Brewer's probably the best person to ask about mountings, and Gary La Pook's probably the best person to ask about Kollsmans, but I'll have a go. Appologies in advance for the spelling.  Usually I work in WORD first to get a spelling check.

Thanks David, You didn't misinform me. The Sextant mount is in a DC-7C, and the mount bears the marking KSH0203, which corresponds with an RAF Mk1 sextant.
Interesting that there should be a KSH sextant in a USA areoplane.  It must have been installed for a UK or Commonwealth airline.

The azimuth ring on the Mk1 sextant appears to be part of the sextant, as per the Service manual for same. Also, the bottom of my sextant mount seems similar to yours albeit a considerably smaller diameter. Perhaps they are easily detached and become separated by accident etc.,
It looks like the azimuth ring off my KSH0203 has been lost at some stage.  I bought it on eBay from New Zealand over ten years ago, so there's not much chance of my complaining now.

My example is partially corroded. The interlock Bowden cable is but a memory, and the hardware to dismantle the hatch cover handle locking block is very inaccessible, without removing the complete mount from the fuselage attachment, which someone already tried and failed.
My mounting is a MK1C for a MK2B sextant, so it might be different.  However, I've just looked down the top with a powerful torch.  The operating handle appears to be connected directly to the trap door.  The locking block just locks the handle until there's a sextant in the housing tube.  The important safety catch is the large black one inside the tube, not the smaller finger catches, which just grip the rings. Theres a fourth safety catch, which only comes out as the hatch is opening to prevent the sextant being removed again with the hatch open.  This means the cable inner must be a stiff wire so it can push as well pull, or have a strong return spring.    the cable outer can be adjusted.  I know that, because one day during pre-flight checks, I pulled the lever, and the hatch oppened without a sextant in the hole.  The technician came and twiddled with it, and it became serviceable again.  After that I never went near the handle in the air without putting the sextant in the lower position first.  It might be worth seeing if you can get hold of any remaing inner and giving it a pull with pliers.  You might have to pull quite hard; I had to push the black trigger inside the tube really hard with my finger to make it go in.  If no inner cable is left at all, maybe you can fasion a hook to contact whatever the cable was connected to inside.  Check first without the sextant in the mounting.  If the hatch opens, check the fourth safety catch hasn't popped out, with and without the first safety catch pushed in with your finger.  Otherwise, you'll need to devise a method of getting it in again, or you'll never be able to remove the sextant once up.  Needless to say, I'm assuming your doing this on a non-flyable aircraft.  Don't attempt to touch a serviceable aircraft without a licensed engineer present, and don't, whatever you do, try it in the air or with the aircraft pressurised. 

Also the actual mount differs a little from the patent drawings. I need x-ray vision.

If I may, I will ask you two further questions, probably stupid ones, as I am not a navigator (ex radio officer, 74-79).

My Kollsman sextant azimuth scale appears to me to read backwards. What am I missing here?
To answer this I'd need a Kollsman Mounting, so I can only hazard a guess.  The azimuth ring is fixed to the mounting.  You can see it through the eyepiece, which is on the back of the sextant.  Imagine your flying north and the ring's set up so you can see 360.  Turn the sextant 10 derees right.  You need to see 010, but the back of the sextant has moved left.  For you to read 010, the azimuth ring would need to have its numbers increasing in an anticlockwise direction.  I could be completely wrong.  I know who'll tell me if I am.

Secondly, the index scale seems to read up to 100, rather than the 90 I would expect. ??
My US Kollsman goes up to 93.13, my German one to 93.22, my RAF Mk2B to 93.27.  You need a bit of leeway if your doing a zenith fix (not that I've ever done one); you don't want to run out of movement half way through a shot.  If yours goes up to 100 degrees, you're either very lucky, and everyone'll want one, or very unlucky, because something's slipped inside the sextant. 

Attached is a photo of the front page of the mk1 service handbook.
If you've got the whole of this book, I can tell you no more than what's in that.  If you've only got the front, Paul Brewer will gladly sell you the remainder for very few UK£s. DaveP

Cheers,

Fergus.