Thanks. That answers my question.
In that first pic above (the one I quoted) I mistook the cylinder wall for a squish band on the piston periphery.
Just wishful thinking on my part.
I'm in a conundrum at the moment trying to help a friend put a nice motor together and we can't find any decent '77 spec S or RS pistons anywhere.
We're either going to experiment with machining a flat periphery into a set of domed 9.5 pistons to suit a set of squishband heads. Or machine an angle into the squish shelf of a set of squishband heads to suit the domed 9.5 piston.