What you have is a First Flight Cover from Havana to Key West, which was completed on October 29, 1927. (It was rain delayed from October 28, 1927, which explains the Havana postmark date). If you want more information, the cover is referenced at the bottom of page 2 at this link:
http://www.nwpl.org/documents/augus...ectronic.pdfAs for the advertising component for the Plaza Hotel (or Hotel Plaza) it still exists and operates today:
http://www.hotelplazacuba.com/hotel_details.aspFinally, as it relates to the addressee of the cover, George T. Street, he was a noted correspondent and cover specialist of Cuba and Central American countries back in the day. In fact, you might very well be interested in this bibliographical record of a letter published in the Scott Stamp Journal from 1928:
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