Alertken..interesting stuff there. The history of how some helicopters were bought over others is as interesting as similar decisions concerning fixed wing types, though rarely told.

I have books on the H-21 and Bell tandem rotor HSL that give hints, but stop short of going into depth.

Interesting that the USMC went with the CH-53 and not the Chinook like the U.S. Army. Both were outgrowth of smaller series, the SH-3 Sea King and the Vertol 107 (which became the CH-46 for the USN/USMC.