Different types of Mullard CV4004?

I bought two pairs of Mullard CV4004 12ax7 tubes off of two different sources. The two sets of tubes look identicle on the inside but have different labels. One set has 'CV4004' with 'KB/QDD' printed under it. The lettering is surrounded by a rectangular box. A smaller box sits on top with the lettering '7727' inside. On the other side of the tube is printed 'M8137' with 'Made in Great Britain' printed underneath.
The other pair of tubes has '000-4004' printed on it with 'CV4004' printed underneath. Below that is printed 'KQDD/K' and below that '81-49'
Can anyone tell me the difference between the two sets of tubes?

Thanks guys
Hi from Hawaii:

I think I can give you some basic info that you might have already figured out.

The M8137 is a military designation, so likely built to higher standard. I have a few.

The other one is a standard issue, manufactured in 1981 (81 in code)

Hope that is "news" to you.

It is news....thanks. Does that mean the Military designated tubes will sound superior? When would these tubes have been made? Wish I was in Hawaii!!!!!

Thanks Terry