To Mark, Glenn & CR members:
I recall that my 1980s Cinelli catalog listed "SL" tubes as Strada Leggera, or Light Road tubing. This was meant for riders under 160 lbs in weight. the main tube thicknesses were 0.9/0.6/0.9 mm.
SP tubes were "heavy" road tubes meant for riders exceeding 160 lbs. in weight. The main tubes were 1.0/0.7/1.0 mm in thickness. I recall that the fork blades and chainstays were also thicker.
The SP tubes were definitely firmer riding. I had a 1980s Torelli frameset (which I sold last year) which was built of Columbus SPX. (Later model SP tubes, earned the "X" designation by adding reinforcing rifling in the tubes adjacent to the bottom bracket.) My Torelli was supposedly built by Tomassini, and it was a cleanly built bike, but was too firm a ride for my slender 150 lb frame.
My 2001 Mercian Strada Speciale, built with Reynolds 531 C (competition) has thickenesses similar to SL, and is a more comfortable and compliant ride.
Andrew Gillis (cool and windy in Long Beach, CA, and more rain due this week)