I photographed these Sikorsky R-4B / HNS-1 Hoverfly examples and a prototype on display at Udavr-Hazy, Ft Novosel, Ft Belvoir, NAS Pensacola and the NMoUSAF.
Vought Sikorski XR-4C 41-18874
This is the prototype for what would be the first mass-produced helicopter. It made its first flight just weeks after the US entered WWII.