I either needed a 26.8mm post to build up the Unknown grey thing, or a 31.8mm front mech to build up the Zaskar, and in the end I ordered a newer M750 front mech from CRC, just so I can hopefully use the Zaskar on holiday. Picked it up this morning, so I'll be fitting it as soon as I get home.
At some point I'll need a longer 26.6mm post for the Diamond Back, but not sure you can get ones of a similar period that are long enough, which would be a shame as the SR Laprade seat post is what it would have originally had and really suits it.
Thanks, I'm hopeful I have everything for a rather mixed up Zaskar build, but I'll let you know if I get stuck!