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 21A5818/POLY and 21A2560/POLY.