@ MICHAEL: May I ask what's the meaning for "SL"? Super Large? :)
Well, the term SL is not clarified yet.
But this could be a hint:
In 1976 the SL watches catalogue includes exclusively steel watches.
Since 1977 bi-color watches and also the early "Yacht Club II" were added,
focusing on quartz movements.
Until 1977 the Club - and SL models were ONLY equipped with IWC in-house
movements. Later models, like the Ingenieur SL ref. 3003, ref. 3303, Da Vinci
SL ref. 3074 and Yacht Club II quartz, also were equipped with ETA based
quartz movements.
Overviewing these models one might again assume, that the term SL changed
from STEEL LINE to SPORTS LINE.