This is what vacuumtubes.net had to say when I asked how they can tell its a 421a and not just a 5998.
"Tung-Sol (and their subsidiary brand Chatham) was the only company to ever produce the 5998 (both the top and bottom getter variants). You'll see them branded with GE, Sylvania, RCA and every other smaller name out there- but they are all Tung-Sol.
The WE421a is rather similar to the TS bottom-getter 5998. They sound similar, they look alike, and they have rather close operating specs. However, the 421a is a vastly superior tube, but considering it's scarcity most people end up with 5998s because collecting a quad of matched 421a is somewhere between unlikely and impossible, and the 'close enough' factor applies."
There was more about history of Western Electric and what not. He didn't answer directly I guess the question but seems to know what they're talking about I guess. I said go ahead and invoice me for it. Time will tell I guess.
They emailed back and explained much further and in detail.
" I'm glad that our reputation proceeds us, but I still like to make sure that we're always known to be on the level. There is one easy way to identify the 421a over the 5998. The top mica and support column has three evenly spaced separators in a triangle.
(((Sorry for the poor photo. We really need a better camera in the office...)))
The 5998 just has 2. Beyond etching, printing, boxes and labels- the internal construction can't be faked. Most of the "421a" tubes on eBay are actually bottom getter 5998s (or sometimes they're even trying to pass top getter variants!). Ask them for a photo of the top of the tube and most of them just don't answer.
The 421a comes with two types of printing:
The first is the well-known heavy yellow paint. It was an early form of thermographics, and the lettering is raised. Not just raised a little- but often enough to tell what the characters are without looking at the tube.
The other is blank. These were left unbranded for commercial sales to radio and industrial manufacturers. Not flat printing, not printing on the glass, not white lettering on the top of the glass... they were heavy yellow or blank. Everything else is rolled on after production.
When you're looking at audio tubes, and especially expensive tubes, you have more than a right to ask about its authenticity. If it is a tube I can't prove I'll give you my reasoning as to why I feel one way or another, and we can compare notes and research. We would rather pass on a sale of an iffy than risk that hard-built reputation."
