Yeah I think even earlier 6.3 volts was also the voltage used for the heating element on vacuum tubes. I think because very early radio's used 6V batteries for

Yeah I think even earlier 6.3 volts was also the voltage used for the heating element on vacuum tubes. I think because very early radio's used 6V batteries for the coils. And a separate high voltage battery for the plates. Would not surprise me if there are roots going back to telegraph.