the only high-end bike that I've ever seen to use a reducing insert in the seat tube is the Cinelli with the classic "fastback" seat cluster they pioneered: these will have a 26.2mm post, (and that's due to the extra material that Cino required in the top of the tube to create his signature stay attachments). There may be others that use a "collar" in the same place if they copied the Cinelli style fastback or made a variation of it, but the most widely known copy, the Windsor Pro, was notable because they did NOT add the extra sleeve, and that's one way to tell a Windsor-wannabe from a true Cinelli.