The Cinelli required a 26.2 seatpost, whereas all the Windsors I've seen took a 26.8, so if it's a 26.8, it's a Windsor or other knock-off. Later Cinellis switched to the standard 27.2 mm post diameter.
There are other visual clues as well. The scallops in the seatstay tops look different Cinelli vs. Windsor, as Chuck Schmidt mentioned a few weeks ago, and I think Peter is referring to below, but that's somewhat subtle and easier to tell with the two types side-by-side.
Greg Parker A2 MI USA
Peter B. wrote:
> The top of the seat cluster, and the post diameter, make me think Windsor,
> but I certainly wouldn't insist I was right. Windsors, as I'm sure y'all
> know, were Cinelli look-alikes, made in Mexico, utilizing the same lug set
> as
> their Cinelli contemporaries.
>
> Regards,
>
> Peter Bridge