Are All Mullards Created Equal?

There is a web site I stumbled upon that had some opinions on all sorts of different tubes. I purchased some Mullard 6922's from the site. I think it is Brent Jesse Recording or something like that. I don't know if he reviews this specific tube but it might help.

I believe the 5AR4s from the 50s to early 60s, built in England and Holland, are most desirable. Fat base, brown base, and metal base perhaps more so. These will be at a premium, but supposedly, they are indestructible.

The Blackburn Plant 5AR4/GZ34/CV1377(especially 1958-62, F31 series) = #1(ie: http://cgi.ebay.com/LOOK-NOS-NIB-f31-Fat-Base-Mullard-GZ34-5AR4-/360331594538?pt=LH_DefaultDomain_0&hash=item53e56fcb2a)

The Mullard, or Wild duck (Anas platyrhynchos[1]), probably the best-known and most recognizable of all ducks, is a dabbling duck which breeds throughout the temperate and sub-tropical Americas, Europe, Asia, New Zealand (where it is currently the most common duck species), and Australia.

The male birds have a bright green head, while the female's is light brown. The mullard lives in wetlands, eats water plants, and is gregarious. It is also migratory. The mallard is the ancestor of all domestic ducks, and can interbreed with other species of genus Anas.[2] This interbreeding is causing rarer species of ducks to become genetically diluted.