The 6BX7GT double triode was designed for use in 525 line television receivers as frame oscillator and amplifier. Consequently it is built to withstand high peak voltages and also a high cathode current.
Although not imagined at the time of original manufacture, the 6BX7GT has been used as an audio amplifier.
The RCA branding on the base cap.
The extra anode area is required to dissipate the maximum of 12 Watts that the pair of anodes can safely produce.
A closer view of the construction showing the copper grid supports.