Mullard 12ax7 dating
The only way I could tell is by looking at the writing on the tube.Original Telefunken printing is very poor and will wipe off easily with your fingers or just a little moisture.
Many prefer these to the Telefunken and are probably the second most sought after brand of 12AX7. The earliest versions from the 1950's have a "treble clef" logo or just the name Amperex (looks like the lettering above).. 1970's versions have the "world" logo in orange printing. The easisest way to tell the difference is to look at the side of the plate in the center.
I really couldn't tell you the difference in the sound quality...I said, buy one of each and try them both.
There are allot of "fake" Telefunken tubes out there, you need to be careful.
6BQ5/EL84 - Standard quality tube E84L/7320 - Premium quality tube 7189 - Industrial Version with a 400 V plate rating vs.
300 V for EL84/6BQ5 7189A - Industrial Version with higher screen and plate ratings.