Girling Master Cylinder Numbers

[TomMull]

I'm trying to identify the Girling master cylinder on my 1973 USA spec GT6 (KF22288U).

I don't know if the cylinder is original.

Here is what I have:

Casting # 646767 B (possibly 8) 03

Green tag # 6868 010N60/40 19HF

The number on the green tag is 6868 010N60/40 19HF. Since I measured the bore at 6874 with my cheap HF caliper, I'm guessing that the 6868 on the tag is the bore (.6868 or 11/16). Am I correct? I think other GT6 masters are 5/8

My standard 3/16 tube nuts fit the ports.

While the bore seems to be consistent, the front and rear pistons seem to be slightly different diameters. Can that be right? The rear piston, which is the one that doesn't pump is smaller.

Your thoughts appreciated.

Tom