The confusion regarding piston rings is probably due to the fact that 3 different generations of pistons were fitted to the SL230 .
Generation 1 with 4 rings ,24 mm bolt .
Generation 2 ,3 ring ,24 mm bolt .
Generation 3 ,3 ring , 25 mm bolt.
Each generation was also available in 3 sizes 82mm ,82.5mm and 83mm . I have researched this after I purchased in error a full set of 83mm Gen 2 MB pistons (for sale at half MB price. ) .
I believe all MB brand pistons were manufactured by Mahle for MB .