After a few months, I finally found the answer to my own question on someone's blog. Link here: http://robertmusic.blogspot.com/2010/06/emission-labs-eml-5z3-revelation.html
"Now, first a word about mesh plates. There are some very shady marketers calling any plate with a hole in it a 'mesh' plate - such as the Sophia, Full Music, and TJ brands. This is complete bullshit. NONE of these are mesh plates. They are solid plate tubes with a bunch of holes punched in them. That is just plain false - mesh plate tubes have exactly that - a true wire mesh woven as the plates. Nobody but Emission Labs has done that incredibly difficult manufacturing process since the mid 1930's.
A real mesh is glorious to look at, but more importantly, the sole benefit of the mesh cannot be realized with a punched hole plate. That benefit is purely sonic. Mesh tubes have a wonderfully organic, euphonic, relaxed sound that is not veiled in any way - open, transparent, yet at the same time possessing a warm inner glow and sense of organic integration that solid plate tubes simply don't have to the same degree. There's something that just sounds very right, integrated and whole about mesh tubes.
That's been my experience with mesh power tubes - and yes, I do have NOS Perryman mesh 45 tubes from the 1930's, dead mint, to compare."
The guy is talking about a 5Z3 tube, but elsewhere on the site he makes a similar comment about 300B tubes. The ONLY true mesh 300B on the market is the EML 300B - all the others are "punched hole tubes" (as he puts it).
Looks like I will have to source a couple of pairs to try.