This was the first S-55 delivered to the US Marines, on 27 April 1951. The US Navy had received construction numbers 55006 through 55015 as HO4S-1 helicopters for evaluation in the anti-submarine warfare role. 127783 to NACA at Langley in March 1953.

According to Joe Baugher the USMC Museum example is actually an HO4S Sea Horse, but he does not offer an identity for it.

Delivered to the Marines on 26 April 1951. HMR-161 deployed to Korea that August. 127788 crashed in California on 3 June 1953. It was meanwhile serving with HMR-163. Photo from: Seaforces-online

An early S-55 landing marines at Incheon (Inchon) in 1952, probably on 1 September. Note that this was not during the major amphibious operation which took place at Incheon two years earlier. Photo from: Seaforces-online

'In 1956, USS Sealion participated in excercises to test the feasibility of using submarines as platforms for helicopters during reconnaissance missions. Marines were shuttled between the sub and the shore on a Sikorsky HRS-3.' This is a HRS-1 on the deck of the submarine. Photo from: US Naval Institute

An early S-55, delivered in April 1951. To NACA in 1953. Seen here with an X-15 drop model. Later to N4781S. Photo from: NASA

The H-19/HRS-1 was the first practical single-rotor utility helicopter and the first in Sikorsky's successful S-55 series. The centre-of-gravity problems of earlier models were solved by shifting the engine to the front and the passenger compartment to beneath the rotor hub. Other innovations included offset-flapping hinges and hydraulically boosted irreversible controls. This example is now on display at the Steven F. Udvar-Hazy Center of the Smithsonian NASM. Ex US Marines 127799.