Outside of piston can be pitted, I'm interested in the screw/spring contraption inside.  Not sure if left or right side is important.  The piston I took out had "R" on it.  And it was on the LHS?  Maybe the "R" meant "rear"?  Should have been obvious, the rear ones have this handbrake screw stuff in it?  The caliper on this year model is marked "16V" on the casting.

The reason I need another is that the new piston I bought does not have the handbrake guts and the supplier said cut the old one open and reuse the guts.  I cut the old one open and found I'd nicked a tag on the spring.  I didn't realise the spring had a tag that extended so deep into the piston.