The seller has put a reply to my questions alongside the info of the sale. The frame size is 51cms (presumably centre to centre) and the top tube is 5 3cms - dimensions that would tend to explain in simple terms the length of the head tube. The seller has confirmed that the tubing is the special seri es supplied by Columbus.
In my haste to out the bike to the List I omitted to read the advert fully. The bike was actually given by Claveyrolat to the seller..hence very good provenance. The seller must have attempted to sell the bike previously but the auction only reached 995 euros..hence the new starting price of 1000 eu ros, but no reserve.
Norris Lockley, Settle UK