The Evergreen museum claims that this is the fifth S-51 built and this appears to be correct, meaning that it was one of the first commercially operated helicopters in 1946 as NC92803 - see other picture. After bankruptcy of the pioneering HAT company it was sold to the USCG as 233, later 1233.

HAT was the world's first commercial helicopter operator. The S-51 appears to be NC92803 which was delivered to the company from Camden, New Jersey, in September 1946. Air mail trials were flown in New York City, Boston and Philadelphia the following winter. A mail van is waiting on the right but we don't know where this photo was taken - do you? HAT filed for bankruptcy in October 1947. NC92803 went to the US Coast Guard as an HO3S, 233. It is now preserved in the Evergreen museum, Oregon. Photo from: Nationaal Archief (Netherlands)