If anyone is interested, got some great responses on the MK1 Mini Forum, technical talk section.

I read that the GMC159 brake master cylinder was fore/aft split and that the GMC160 was a diagonal split system but used the same casting so externally, one can't tell the two versions apart.  And that the upper pistons were different, not interchangeable.  I'm in the process of rebuilding one using parts from a couple of different salvaged MCs.  Does anyone know how the upper pistons differ and how to tell them apart?  Or whether it makes a difference.  The brake lines go out of the MC to the brake proportioning valve, which routes the fluid fore/aft or diagonally, right?