ObUsage - that's ECC83 in BrE.
>"E" - 6.3 V heaters.
>"C" - triode
And the first digit '8' denoted its B9A base. Only the final '3' was available to indicate variations on the ECC8* theme.
It's rather fun that it's called "12" and "E" and both are right.
Eh? Oh, you mean about the series-connected heaters with a centre-tap so they could be used in series for 12.6V or in parallel for 6.3V ? I suppose that's "fun" for a certain value of "fun" :-) Possibly slightly more "fun" than the dreaded U- and P- series valves/tubes, often used in low-end consumer kit, that had a specified filament *current* (P=300mA, U=100mA IIRC). Allowed the filaments to be connected in a series string and powered directly from 240VAC with the help of a huge, hot and unreliable ballast resistor. Happy days.