Mullard Tube Code Help

Hey guys! Could someone please help me identify the following Mullard Tube Codes. These always really confuse me. Thanks so much for your help!!!

"F33 B8K3"

"F31 B91"
F33 = GZ34/5AR4 B8K3= Blackburn '58 or '68 Nov third week

F31 same tube
Samhar, Thank you!
The F31 was made in Holland and relabeled, I'm told. Glad I could be of help.
Samhar... that's some pretty impressive knowledge you are sporting there! How do you know that?
A little additional info. F3 is the tube type. 1 (and 3) is the change code or version of the tube.

B91 = Blackburn, 1959, Week 1

In the early 60s they went to a 4 digit code that included a letter for the month. So,

B8K3 = Blackburn, 1968, November, week 3

If the B is etched into the tube, it was made at the Mullard Blackburn, England plant. The painted on labels were often changed. That is, a Mullard made tube could be painted with an Amperex or RCA label but the etched codes always tell you where and when a tube was actually made.

For more information just google Amperex tube codes.
I'm getting better with my keyboard, almost worthy of AA!!!!
Tomcy6, thanks for your additional information. I found those tube code pages on the web, but still couldn't decypher them. Seeing how you used the code and what it means now makes sense...thanks again.
You're welcome Mofi. I have a pdf of the codes that someone cleaned up that is much easier to read and understand than the ones that are copies of old booklets. I can email it to you if you send me an email.