[Arran]

Actually if you read the spec sheets on most loctal tubes, like a 7C6, 7A8, 7B7, etc. the filament current is only 150 ma rather then 300 ma, so they were not just another 6.3 volt tube with a different base, this was why Philco used them in many AC/DC radios during the 1940s. I think that the main reason for denoting loctals with a 7 or 14 rather then 6 or 12 was to avoid confusion with octals or older glass tubes, a 6B7 is an RF pentode, double diode, a 7B7 is just an RF pentode. The curious part is that they didn't do this with battery operated loctals, they added the letter "L" to differentiate them from the octal types.
My understanding is that Sylvania supposedly developed the loctal types for use in car radios and other mobile uses, and there were car radios that used them, but most seem to have ended up in AC sets. The idea being that the tubes would not get rattled out of their sockets, but I think this was the case of a solution that was seeking a problem. I have had several car radios, some used octals, many used seven and nine pin tubes, and I have never found one where the tubes become unseated and was rattling around inside the metal case.
Regards
Arran