Rover piston TAM2048 stamped 21962 with ring sizes 1.501mm and oil ring 4mm but some pistons have been stamped 21963.
-Does this get confusing?
So lets clear it up; the easy way to identify the piston and corrrect rings are as follows-they are made from the same piston slug with same dish but the high compression version is easily identified by the lower oil ring being 3mm wide and the top ring gap to the crown is 10mm deep.
The low compression version has 4mm oil ring and a shorter length top ring gap of about 7mm to the crown/top. The gudgeon pin is moved on production to give the higher compression ratio and lowering the rings giving a larger top ring land for strength. Rings for 3mm lower oil ring type are
R41570-40.
if you have 4mm oil ring use
R35960-40.