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This reply was very helpful in removing the bubble illumination assembly from my A-12 sextant. Especially the details in.  http://jphtechnologies.wordpress.com/2009/12/20/137/

To be slightly more specific for the next person, I removed the clear natural light window, put 1/4" long bits of rubber tube on the ends of the jaws of some needle nose pliers, and slowly unscrewed the bubble illumination bulb. I then held my A-12 in some soft jaws with the bubble assembly down and soaked it with WD-40 for a week. It still didn't slide out as the book describes.

I chipped the corner off a piece of pine, shortened it to 40mm, whittled the working end down to about a 5 x 9mm oval so it would fit down beside the fragile bulb holder, worked it in through the light window, and pried with a medium size screwdriver against the frame and the wood. The entire assembly slid out intact. There were no batteries left or I'd guess it would have never come apart.

I had removed the rheostat plastic knob after loosening the hex wrench set screw but it turned out that I didn't need to. 
I don't see any corrosion, or expansion, on the zinc(?) bulb holder/end cap or on the thin aluminum body. I guess it was just a combination of the perversity of inanimate objects combined with not having anything to grip. 
Regards, Noell