I went to look for my spacer to measure it, but alas I have lost it.....but I did measure my seatpost on my AO-8 and its 25.4mm; my frame is older with Aztec lugs so YMMV. The slot is not completely closed up and the post is an SR LaPrade.

Oh, and to juvela 's comment, the original seat post was indeed not stepped and I suffered the dreaded seat slip due to the missing step....luckily no injury but there was a circular dent in the bottom of the leather as the saddle bottomed on the top of the seat post.