NavList:

Bob, that's very kind of you - I would appreciate the measurements - particularly for the height of the washer.  If I can't locate one I'm going to try casting one out of epoxy and graphite powder - I figure I'll just wax up the bearing and put a tube of some sort over top of it to make the mould.  Then cut the finished piece to the right length and machine out the underside.  Think it should work ok.  

It's an extra sextant for me too - I've got a 1976 Freiberger drum sextant that I've used a bit offshore, but I couldn't resist this one when I saw it for a good price (even if most people would say two sextants are at least one too many!).  I've also got a 1942 Husun, but it's more for decoration - I'm not keen on the little mirrors.