Do you mean P38 diff?
Because they do not fit the older rover axle casings completly different. Series and RRC and Disco are all the same nose length. However they can differ internally slightly 2 pin 4 pin late or early type 10 or 24 spline, metric & imperial and are different ratios 3.5 4.7 to 1. All will interchange aslong as the ratio and splines are the same strength depends on the internals.