My understanding is that the "Super Single-Six" has fully adjustable sights, vs. the original windage-only drift adjustable sights.
I believe "Convertible" was added to the name for the revolvers available with two cylinders, such as the .22LR/.22 WMR models.
Somewhere along the line, the "Super" Single-Six nomenclature seems to have been dropped, as I don't see it currently used on the website.