There is the Schwinn expert called metacortex that knows everything.
I think that is his handle both here, there(other forums) and everywhere.
Like Sgt Schultz, I know nothing but I believe 1967 is the first (s) (s) twinn-stik stem mount for ten speeds.
The five speed model Collegiate also first got the (s) single-stik on the 1967 model.
You are correct about an easily visible change to the twinn-stik occurred around 1974, I think it was but I do not know exactly.
You can see that the twinn--stik levers in certainly the 1967 through 1973 era have the (s) (s) small odd shaped ess somewhat closer to the shape of ess as seen on the logo of rockband, KISS.
----The 1975 on twinn-stik levers have (S) (S) where the ess is larger and more normal looking, resembling a capital ess.
Ditto for the 5 speed Collegiate & Suburban single-stik lever. 1967 through 1973 Collegiate has (s) and I think some if not all 1974 Collegiate models have (S).
1970 was the first year for the Schwinn Suburban, so the 5 speed SUBURBAN of the 1970 through 1973 era have (s).
In 1974, Schwinn chose to use a Thumb Shifter on the 5 speed SUBURBAN.
In 1975 Schwinn changed the 5 speed SUBURBAN back to the single-stik stem mounted lever which in 1975 had the (S) instead of (s) as had appeared on the 1973 model.