540 rotor system
Bell 540 refers to the rotor system, not a model of helicopter. The -10 US Army Pilot Operator manual, whatever it was called, can't remember, used "540 rotor system" to denote the big barn doors hung as rotor blades on the UH1C and AH1G.
As to getting out of the air conditioned cockpit of an AH1G in preference to the unairconditioned UH1C, comfort was not a concern for me in RVN, survival was. The Cobra was not as survivable as the UH1C when various scenarios cropped up.