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I agree with everything else you said, but the Svelto is wrong. I worked in a big Raleigh shop back then, assembled a ton of Super Courses (maybe literally!) but never saw one with a Svelto. That I can recall anyway. I think I would remember it if I had.

The catalog spec, and what my '71 came with, was a Huret Luxe ST. Some people say that is similar to a Svelto but I say not so -- they're just about backwards from each other in terms of their motion. The Luxe is more similar to an Allvit, in that the movable pivots of the parallelogram are at the top, with the fixed pivots at the bottom. So it shares with the Allvit the nice path of the upper pulley, which follows the cone of the freewheel, keeping a more constant gap from jockey wheel to sprocket teeth. Theoretical better shifting, but in reality they were finicky and prone to bending or jamming up with road grit.

The Svelto was a much cheaper model than the Luxe (as the name Luxe implies), with dull plating, low-quality bolts etc, and an unsophisticated action that does not follow the shape of the freewheel well. The Luxe was basically a fancier Allvit, but it wasn't really much lighter or better in any way that matters. Just (arguably) nicer looking.

I like my Allvit on my grocery-getter, but I've had to rebuild it twice. (Not too bad for an almost 50-year-old mech I guess...) The Luxe on my Super Course went away decades ago, so I won't remember why. I probably replaced it with a Suntour V. That bike has been through so many 're-imaginings'. Since the early-'80s, it's had a '50s vintage Sturmey alloy-shell FW (4-speed IGH), that I more recently converted to be a 5-speed (like an S-5, tho the hubshell still says FW Alloy)

I guess the switch from Huret Luxe to Simplex Prestige on the SC was jumping out of the frying pan and into the fire, since both of those shift well until they don't. But at least the Huret is rebuildable. Nothing you can do with a Simplex after the delrin cracks.

MarkB in Seattle