That was the 3rd Edition version. Turns out my memory was still fogged though.
In Original and 2nd Edition, SS made you immune to damage from physical attacks, for a certain number of attacks or attack sequences (claw/claw/bite/tail swipe from a dragon would be an attack sequence). It had nothing to do with AC.
http://pogromisland.com/html/RulesForge/Stoneskin.htm
