I think it's the same as on my 1986 Raleigh Road Ace - I'll have a look tomorrow. It's one of the few parts on my bike that isn't Shimano 600. I saw what was advertised on eBay as a 600 seat post, but I wasn't altogether convinced.
My bike is a 21 1/2" frame - a bit too big for me, but the smallest Raleigh produced at the time - and the fluted part of the seat post is almost entirely sunk into the seat tube, which is a pity because I think they do look rather stylish.