Paul -
I *LOVED* those old Superwounds, although I think I only had them on an Alembic once. They were a truly rare treat which hasn't been around for quite a long time.
Perhaps more clarification on the differences is appropriate. Bass strings have a straight gauge steel core wire that is wrapped with the winding wire that you feel. Because there are practical limits to how heavy the core can be before it becomes a stiff rod instead of a string (!), each manufacturer picks a different mix of core wire and wraps to get to a particular size. A roundwound low B string often requires two sets of wraps to get to the big diameter and that's part of why they often sound bad. I think even heavy low E strings may have two wrap layers. Flatwounds have multiple wraps as well.
On a regular string, the wrap wires extend virtually all the way to the loop around the ball end and the full diameter of the string passes over the bridge.
By contrast, the Superwound strings were exposed core. In this design, the outer wraps stop before the bridge and only the center core wire passes over the bridge. When you do this, the string is obviously much freer to have pure vibration. This is how the lower strings in a piano are made and, in fact, Superwound referred to them as piano-string design (PSD). All four strings in the set had this bare core design. The sounded great, but were expensive to make and, worst of all, every different kind of bass has a different length between the bridge and the ball end stop, so many different sets had to be manufactured (Fender, Musicman, Rickenbacker, etc.). They also had a variable length set which had no ball end on the bare end and included a set of adjustable ball ends which you moved to the position you needed, then tightened with a setscrew!
The Superwounds were the original factory equipment for the early Steinberger L-2 bass. I played Steinberger for many years and still have three L basses. In addition to double ball ends (no headstock!), the witness points on the bridge were knife edges. I still have a few sets of Superwound 707s from that era. When you use modern strings, the wraps pass over the knife edge and it's not as nice to tune as the knife edge bumps the strings up and down as you tune.
What you usually see these days are what I would call a taper core or taper wound. On these strings, they stop the outer wrap layer before it passes over the bridge, but the inner wrap layers are still there. Since the string is a little thinner, vibration is improved. Normally you only see the tapers on the low B and possibly E.
I'm not sure quite what was going on with the Superwound company. It was always publicized as if it were an independent company, but it had the same address as Rotosound in England and James How, the designer of the Rotosound strings was associated with the company as well. In addition to roundwound PSD strings, they also had a very early pressurewound string called the Linea finish which were the nicest strings around. Pressurewounds start out as roundwounds and are squeezed in rollers to have a slightly smoother finish but maintain the sound quality of roundwounds (this sounds like the description of the Alembic strings in your followup post). The current Rotosound Solo Bass strings are this same Linea finish.
