So, is this your car?
My 23i springs have S1 printed on.
When my car rear end was clunking the first thing I checked was the springs, but were fine.
Eventually just needed turning 90 degrees and rubbers coated in talc to stop the noise.
I think the barrel spring design is very poor as the loads are not vertical like a normal coiled spring over strut, as is the front end.
Leading to a point load on that small diameter at the bottom.