Use the Girling reference number on the slave cylinder.

There are two types if the slave cylinde is mounted at 12 o'clock on the belhousing. These were fitted to Austin A60 Westminsters as well as Formula Ford 1600s.

They are available from Girling and Willwood as well as thise of Chinese or Indian manufacture. Personally, I'd avoid the asian ones.

I hope that this helps.