The 6ER5 was in fact a frame-grid VHF triode, not a tetrode, and was evidently improperly named when it was released, as explained here:
The Mazda 30F27 was a frame-grid VHF tetrode. It had aligned grids and possibly followed the Harries principle in order to minimize the incidence of kinking. I haven’t seen the curves for it though. I think that Sylvania also had a double VHF tetrode (on a decar base) for FM front end applications. Here is the background to the Harries valve:
The above-mentioned Wikipedia article suggests that the Harries valve was an alternative development aimed at minimizing tetrode kink. But as I understand it, the Harries idea of a space-charge virtual suppressor grid was integral to the beam tetrode as developed by Osram, and also taken up by RCA via its information exchange pathway with Marconi.
In the RMorg article, the “side-plates” commonly used in beam tetrodes are referred to both as “beam-confining plates” and “beam-forming plates”. The first is more accurate, I think. The sheet beams are formed by valve geometry, namely grid spacing and alignment of the screen and control grids, and do not require the presence of the side plates. Rather their function is to keep the sheet beams away from those parts of the valve, where due to the presence of support rods, changing geometry at the ends of elliptical long axes, etc., the low-potential virtual suppressor grid is less well-defined.
Cheers,