These pictures show a real oddity. The tube at left is on an octal base and is branded "United Electronics 6B4G". The tube at the right is a Western Electric VT-25 (transmitter-rated type 10). United was an old US maker of transmitting and industrial types. They NEVER made receiving tubes. Someone else owns the brand name and has used it to rebrand many types. The tube at left is simply a rebased VT-25 made either by Western Electric or National Union. While VT-25s by Western Electric are pretty valuable now, they were cheap surplus following WWII. The feature that most gives away the fake as a Western Electric tube is the use of rounded triangular mica supports.
I also want to say something about "mythical" single-plate 6A3 and 6B4G tubes (the 6B4G is a 6A3 on an octal base). In short, there is no such thing as a real single plate version of either. Most commonly seen single plate 6A3 tubes are actually renumbered VT-52 (not VT-25, as above) tubes. Again, VT-52s were extremely cheap on the surplus market, and they had absolutely no civilian use. Remarking them as 6A3s made them saleable.
The biggest problem with using either a VT-25 or a VT-52 to replace a 6A3 or 6B4G is that the VT-25 has a 7.5V filament and the VT-52 has a 7V filament, while the 6A3/6B4G has a 6.3V filament. This means that new VT-25 and VT-52 tubes would probably function, but would have a markedly shorter working life at the lower voltage.