It does indeed look circlip like, but it would never have got through the tiny gap between piston and bore in that condition, looks way too untouched for that.
I've never had a SINGLE drop of oil from a sump on any car, simply use sealant, hylomar etc, whatever you have on the threads, stops 100% of the leaks, even caused my astra sump to leak, due to rusting so got a new one.
tabetha