Yep, sorry Phil, you are right.
I have an old Bock tap that is 6.4 X .6 which is the #5 nib (if I remember rightly) I also have an old 6.5x.5 as well. Checked sizes and i have it down as th 6.5x.5. Just shows that checking sizes should be done rather than checking the tap and die box.
Not done any for a ling, long time, with all this "revival" going on I will have to make some time me thinks..