Anyone familiar with Mullard 12AX7 10M tube ?

Well when these Mullards were made you didnt have components that really beat the tubes as some do now. All depends on the circuit I think.

10M are sort of guaranteed for 10000 hours. Well, other Mullards might last just as long if you are lucky, but I don't count on it. I count on 5000 hours. So far I have about 2000 hours on the narrow long plates.

A number of tube manufacturers used special designations for what they claimed to be higher quality examples within a tube family. My guess, at most, is that they may have factory tested and given this badge of honor. There is no way the 10Ms, Gold Brands of 5 Star tubes were made differently than their more common brethren.

The range in Mullard sound is based upon production periods in my experience. This is true of many tube manufacturers.

I can't say because I only installed Mullard 12AX7 narrow long plates in my VAC's Avatar SE preamp section and they sound excellent. I did read that, yes, there is a  difference, there is quite a range of Mullard sound.

If I may ask an additional question, I’m not trying to hijack your thread. Of the NOS Mullard 12AX7 tubes , is there much ( any? ) difference in the sound characteristics of the sound. I just picked up a pair of tube mono blocks and they each take 1 - 12AX7 and 2 - 12AU7’s. I’ve never rolled 12AX7’s but in rolling 12AU7’s I tried a standard Hammond labeled NOS Mullard 12AX7. Thanks, to the Op. Respectfully, Mike B. 

No!

I see. Yes, it might be equipment dependent to a degree.

I had a pair that I used in a McIntosh MC275 about 10 years ago and I remember thinking to myself that they fell short of expectations.  Very small sample size... YMMV.