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Fergus Kavanah wrote:  What is equally curious is the lengths to which KH went to with the interlocks to prevent opening the hatch without a sextant in the tube, when Kollsman didn't bother with an interlock at all. I know the tube diamater is greater on the KH, but still.  The KH interlock is built like a brick outhouse. I spent a long weekend attempting to extract the seized locking bolt from the slot (unsuccessfully), before reluctantly breaking the mechanism from the housing. Subsequently fitted the sextant only to find that somewhere in the very distant past, the entire mounting had been removed and re-installed 90deg out of the correct orientation, so the graticule could not be centred correctly. When I removed and re-oriented it, all was well, and at the correct orientation the removal of the locking bolt would have been fairly easy. Ah, hindsight is 20-20.  You will have gathered from the above, David, that the Mk2a sextant can be fitted in the Mk1 mounting, except the azimuth ring is not the correct diameter.  The sextant protrudes a similar amount as the Kollsman in the 707 does, so I assume it is more or less correct. (See attached photos). A question, if I may. The DC-7 has a transparent panel to the left of the sextant mount, with a relative bearing scale marked at 30deg intervals. What was the purpose of this ?.

Hello Fergus.  I hope your not going to try and fly in that DC7.  You’ve clearly shown that a Mk2 Smiths sextant will fit into a Mk1 mounting.  I thought I’d try a Mk1 sextant into a Mk2 mounting.  It’s OK diameter wise, but either the bottom of the mounting tube is a couple of mm too long or the collar on the top of the sextant is a couple of mm too high for the bottom locks to engage to hold the sextant in place when the sextant is in the up position.  If you hold the sextant in the up position, it could do with sticking out of the aircraft a couple of mm further.

The scuffing on your Mk2a sextant head is caused when people try to insert the sextant into the mounting without first squeezing pinch grips on the mounting, a very naughty thing to do.
I can’t really help on the transparent panel in the DC7; I’ve never been in one.  This chap might know https://www.youtube.com/watch?v=gHrZqcjG_O4  .  Watch between 45 and 50 seconds from the start.  He’s using an early Kollsman by the way.  DaveP