All MK3 minis to 1978 except Cooper S MK3 which used 21A1774.
Fitted mainly to cars with 3/4" larger bore rear wheel cylinders.
The correct regulator valve for all MK1/2 mini plus all Cooper S inc MK3 is
21A1774 where most models had 5/8" bore rear wheel cylinders.
The only difference however is in the poundage of the spring, 37H2009 which is weaker for this version to compensate for the change in wheel cylinder sizes.
See
MS72 for adjustable type. The stronger spring in
21A1774 is available as
27H6620.