The U.S. Navy ordered four S-51s "off-the-shelf" from Sikorsky in late 1946 for use in the Antarctic and Operation Highjump, placing them into naval inventory as the HO3S-1 [REF].
Thirty-nine additional specialized rescue helicopters were built, as the H-5G, in 1948, while 16 were fitted with pontoons as the H-5H amphibian in 1949 [REF].