Ignore the diagrams. They are frequently wrong. Beam plates act something like a suppressor grid and are usually connected as such, so they are so treated in some diagrams. I've seen all kinds of crap printed in old tube manuals....they were usually printed in a hurry, so often contained errors. Most EL34s and EL84s are pentodes, and the other common power tubes are almost universally beam tetrodes.

>I recently saw some "winged C" EF86s. I presume by your post they are, but are these the same tube as the older "S" logo, and not new production?

That was the last EF86 in production anywhere. They MIGHT be making new ones, but I doubt it. The market is miniscule, only a few thousand per year at best. Most end up in microphones and in a few Vox guitar-amp clones.

>should have listed the 6BG6 under the "cult tube" heading, as this is where it's usage is seen.

Yes, please, and remember that there are lots of cult tubes, but very few types still in REGULAR manufacture. What goes in guitar amps dominates manufacture today. Hi-fi things like 300Bs are way down on the importance scale, but have some "snob value". Everything else is probably headed for oblivion, sooner or later. You can get 120 watts of really clean power out of only two EL509s, but nobody cares at this point. Most OEMs of high-end tube audio are conservative, cowardly, cheap--and most are willing to go solid-state at the drop of a hat. As I learned at Svetlana, you must give them what they want--and they usually want 6550s for 1972 prices!