Charles
Have a read here
http://www.germanlook.com/Forums/showthread.php?t=4595
There was a large discussion on the early single pot 944 brakes vs the later 4pots with respect to the master cylinder.
The general verdict was that the single pots require the split bore (23 / 19) 944 master cylinder and the 4 pots are ok with the bug 19mm one.
There are always those that will be different - and s1fter has a variation on this- I'll let him explain it.
Cheers
Jeremy