Marples Hibernia Works brace restoration


I acquired a classic Marples Hibernia Works Sheffield brace along with a dozen or so bits a few months ago.  Although this is not the beautiful Ultimatum model, it does have all brass cladding, ivory inlay, ebony handle, and terrific brass medallion.  It's all in great shape and totally usable except for the latch mechanism to hold the bit in place. 

I've disassembled the hexagonal brass piece that holds the bit to clean it and attempt to replace what I perceived would simply be a spring but am a bit baffled about what is missing besides the spring because there is an additional unfilled, threaded screw hole for which only the very bottom of the screw is visible externally.

Hence I'd really appreciate it if someone would be willing to share a pict or a reference to a pict or even their experience in restoring one of these of the intact mechanism so I can fully restore and replace all missing parts and not just guess at this. 

Also, anyone have any suggestions about how to polish the circular ivory inlay in the ebony handle without destroying the patina of the ebony?  (BTW per a web suggestion, I've tried a bit of Crest and Colgate with a small toothbrush to no avail.  Perhaps I'll need to try a tooth enamel whitener - LOL!!)