Brake Master Cylinder size query

Heya,

Think I've foobared by M/C when bleeding the system on my standard 1985 GTi Cabby.

Now I was going to pop to GSF tomorrow for a new one - but not sure which one I need; 19mm or 20mm?

I know the mk1s originally had a 19mm cylinder, but the mk2s 20mm - did VW keep shipping the cabbys with the former, or change to the latter when they started building mk2s? :dontknow:

Cheers,