Interesting find - as you seem to have something slightly unusual there.

From my research, there was an S12-S, and an S12-S Limited. The Limited was slightly higher up in the pecking order (just below the America, a well respected bike). Compared to the base S12-S, it had slightly better components, bar end shifters, but only 2 chainrings (12 speed). The basic S-12s had downtube shifters but bikes after '81 had the triple front for 18 speeds, like the later America. Both were respectable for the day at around 25 lbs.

In your picture you show a LTD with a triple. I didn't think they ever came that way, hence my curiosity...

I have a brown 1981 S12-S that is a work in progress right now. I'm changing it over to bar ends from the OEM downtube shifters.