The Mini's rear subframe is rubber mounted. Polyurethane (stiffer, much longer lasting) bushes are now available for the rear subframe trunions. Cars manufactured before 1976 can use the same bush (2A5818/POLY) at the front and rear of the subframe, 8 per car.
Starting in 1976 (MKIV) the front trunion bush was enlarged, that bush (21A2560/POLY) is now available in Poly. These later cars use 4 of each 2A5818/POLY and 21A2560/POLY.