Search on google for girling cylinder, but I think Wilwood sells them.

I'm not sure what the sifnificance of a Girling style is, but they all use them. Originally it was a Lucas Girling master cylinder that was used on just about all British cars from 50's-80's I guess. Maybe the important part is that it has a threaded port for the inlet so it can handle pressurized input instead of the simple snap-in type made for a reservoir.

-Sean