If you look at a Brandis sextant, and many others, you will see that they were painted. I've attached a bad photo of one here.
The one I own is also black, and I think you can in fact see black in the box where the supports meet the sextant. I'll have to get some better pictures when I get back to America after the 9th, but I'm pretty sure that simply looking at the box you could easily come to the conclusion that "sextant being old fashioned and probably painted over with black enamel."
One thing that I find very interesting, and a nuance that many miss, is that Gallagher specifically describes the numbers on the box as "Sextant box has two numbers on it 3500 ( stencilled ) and 1542-" - Note that one number is "stenciled", and the other is not. He doesn't say how the other number is presented, but what is interesting is that in the Brandis boxes we've looked at, including the one in the attached photo, and the one I own, the Brandis number is indeed stenciled, but the USNO number has been stamped or impressed into the wood. See this photo from the ameliapedia
http://tighar.org/wiki/File:Numbers09.jpg.
The fact that this fits perfectly with how Brandis sextants of the USN were numbered, - one number stenciled, the other not, is very compelling evidence to me that the box Gallager was describing was in fact a former (?) USN Brandis sextant box.
All we have to do now is connect that Brandis number with AE / Fred Noonan. Simple huh? Well, he was known to carry an old fashioned sextant as his backup "preventer", and he was lost in the area....
Where are those archives?!?
Andrew