Gazelle Trim Trophy's seatpost size

I am having the same problem with a 1950's Claud Butler and have tried 27.2, 27.0 and am about to move on to 26.8 (you dont happen to want to get rid of the one you tried do you?? esp if it is black)

I've got the same problem, but have no spare seatposts to try it with. Give me a shout when you find out.