it is most likely 27.0. seatpost is too new nayway, you need the record model
or this one
http://www.ebay.co.uk/itm/Vintage-3ttt-Criterium-Seat-Post-27-mm-Made-Italy-Used-205mm-/120836555088?pt=LH_DefaultDomain_0&hash=item1c226b8150
@Mr.C started
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it is most likely 27.0. seatpost is too new nayway, you need the record model
or this one
http://www.ebay.co.uk/itm/Vintage-3ttt-Criterium-Seat-Post-27-mm-Made-Italy-Used-205mm-/120836555088?pt=LH_DefaultDomain_0&hash=item1c226b8150