At some point near the end of the era of lettered exchanges, New York apparently ran out of decent names and inaugurated the nonsense exchange of TT which served City Island. (Prior to that the exchange there was TUlip - no idea why they didn't like such pretty flowers.) There were plans to introduce other scrambled letters like YX or HN, etc. The bright boy who thought up that scheme is probably now working as a parking lot valet in Atlantic City.