Steve, it certainly would be very interesting to have more info on the pre-war EMI tubes!
No data book I have (but I don't have that many...) mentions the 6/4.
But certainly data must be in possession of someone on this forum.
Let's hope something turns up!
Perhaps "Catkins" (on this forum) might have a relevant data-book. He has posted this photo of data on the 3/1 and 3/2 (I hope it is OK to mention this here):
Jac