Thank's Neil! That's a rather peculiar size for a seatpost, at least for a steel frame. Probably why we never thought of it. Have been getting by with diy shims and a USE CF Alien post up till now. Shame they don't make any 25 to 30,4mm shims as these take the edge of bumps and road buzz a lot better than the larger versions.
Edit: Found one
, was looking at the wrong type of shims.
I don't know about other years, but at least for 91 (and probably holds true for 92 - don't think much really changed for the steel frames between the two years), the Axis frames, using OX tubing had larger diameter tubes - certainly in the seat-tube - I really should line up my 91 Axis and 91 Apex together to compare - but looking at my Axis frame in isolation, does seem that some of the other tubes are larger diameter, too.
So my 91 Apex had a 27.0 seatpost - quite possibly the same for several of the models below too.