and i thought you knew engines lol , nah it means the mains as you said are different and unlike a s2 which has 1 solid and 1 grooved shell the s1 has 2 grooved so the big ends see oil pressure all the time unlike the s2.
And there not available anymore so you have to buy 2 set and use the grooved from both as the solid shell doesnt line up with the tab.
The s1 valves were also sodium filled or cooled ,tbh the whole engine was much much better the worst being the last engines h plates etc.
Tim