Well, your '80 bike has the older version flywheel and clutch, but as far as I knew they did have an "S" mark too, but I could be wrong. On my bike ('82) the "S" Mark is actually on the same little steel protrusion that the "OT" mark is on, it is just a little bit below the OT mark. With the bike in 2nd gear, plugs removed, I can roll the bike and see the "Z" (full advance mark) and then roll it ~1 inch further on the garage floor and the metal tab with the "S", and just above that, the "OT" marks are. The "S" and "OT" marks are only about 1/4" apart. The timing of the bikes stayed at 6 degrees BTDC on all these years.
To set the basic timing @ idle to have your ignition points timing correct you'll have to find the "S", though if it seems to be running pretty smoothly at idle maybe you're OK leaving that alone for now and just check that the "F" or "Z" mark at full advance is centered in the hole when you hit 3500-4000 RPM.
I tried to just take pics of the marks on mine but it is a bugger trying to get a camera lens in there and to focus on the hole and not obstruct the flash. Just when I thought I finally had it, the #$%^ battery died inthe camera
