NavList:

 Geoffrey Kolbe you wrote:

"it is an octact as the scale only goes up to 90°."

It might be worth you taking another look at the arc of your 3.5” sextant Geoffrey.  There are many photos of the Hughes 3.5” sextant (Stores Ref 6B/177) on-line, but very few in which the scale is clear enough to read.  However, the few clear photos definitely show a 120 degree scale.  That said, this doesn’t necessarily mean that the index arm could move over the full 120 degrees.  It’s possible that the index shades (which look like standard Hughes shades) or the horizon mirror got in the way, which is why the certificate only has corrections up to 90 degrees.  

You also said: 

"The scope seems not to be original" 

Yes, I agree, the telescope on the eBay sextant is non-standard.  A clear photo of a Hughes 3.5” sextant can be seen at  ttps://auction.catawiki.com/kavels/21765857-united-kingdom-henry-hughes-raf-flying-boat-s-sextant-1945 if you can get beyond the “Sold” caption by careful selections with the cursor.  However, there’s also a photo in “The Sextant Book”, possibly of a younger model, which shows a different telescope again, possibly the yachtsman’s or lifeboat model.  DaveP