none of them is a true systems with a wide a selection of lenses, backs,
Beg to differ. RB67 uses no batteries (except a zinc-air button cell, replacement for a mercury cell, if you have a metered prism). There are a range of lenses from 50 mm to 500 mm, macro tubes, tele-extenders, three viewfinders (waist level, chimney, and prism), two with option of metering, plus sportsfinder, and they'll accept virtually any 2x3 Graflok accessory (Graflex roll backs, 2x3 Grafmatic, there's a Polaroid back, if you can find peel-apart film, and there have been at least two Instax backs), as well as 77 mm screw-on accessories. There's even a motor drive film back. Also at least three different auxiliary grip options, two-headed cable release for long exposures using T setting -- I'd call that a system. Surely as much so as a Bronica SQ series, competitive with a Hasselblad 500 series.
Bulky, heavy, yes. Robust, you betcha. Format versatile like none of the other options (I can shoot
four formats of roll film on mine). Shutter in the lens, so if one quits, just swap lenses and move closer or further away.
And still pretty reasonably priced. They've gone up a bit since I got mine two years ago, but still far below Hasselblad or even Bronica.
Oh, and introduced before 1970, that's your half century.