I never thought much about it. I'm a classic III. That's actually my favorite vintage Kirby anyway, because it's the one my grandmother had when I was growing up. That one, and the green 1975 seats best she had, were the start of my fascination with vacuums. That's all I ever played with when I was there.
An interesting read for sure. Inaccurate however in that there was never a model 506 or 507 after World War 2. And what of the elusive R series? Having the switch in the handle would've been an added convenience for sure. Kirby makes no mention of them, but I know they were sold at the same time as the C series.