NavList:

The inspection certificate is a company inspection certificate, not an independent inspection.  We can therefore safely date its manufacture to just before its factory inspection, or 1957.

As to value.  Condition and functionality are everything, as this is not a particularly rare instrument.  If its useable, serviceable and not dropped on its head, its a few hundred dollars.  That's a guesstimate consistent with that online auction house that we all know, eBay.  I suggest you simply go to eBay, find your model sextant and look at completed auctions for your model sextant.  If the optics are wrong, the instrument dropped, etc, then unless someone is looking for parts, the value drops dramatically.  

Frankly speaking, this is a good time to buy a used sextant.  Those who use it aging out of the system and others are simply abandoning Celestial Navigation.  Consequently, the sextant that goes with it are being abandoned too.

The converse is also true.  Not a good time to sell, as there are literally hundreds of sextants available every day on eBay, maybe just not your model.

This absolutely does not apply to rare, unusual or otherwise desirable instruments.  

Brad