Michael, I am under the impression that the center carrier of many BMC automobiles are interchangeable. My A30 has a 5.37 gear. I have purchased the more common 4.22 from a sprite to replace it when I upgrade to a 948 engine. The new differential bolts in. The only issue is the filler hole is located in the front of the original unit and in the back cover of the new unit. I will be adding the filler hole prior to installing the unit.
FYI Austin A30s Saloons from chassis number 43849 forward have the 4.875 gear you are seeking. I am willing to bet that A35s have something closer to 4.22.