The 185BTA is a power valve with a 25 Watt anode dissipation and 5.5 Watt screen dissipation. The peak anode voltage is 10 kV. It was designed for TV Line scan output. The 185BT had a maximum peak voltage of 8 kV. In use it would be biased with a 140 Ω cathode resistor.
The high peak voltages involves are why the ceramic plugs have been added to the supports as they enter the mica discs. Note the screens top and bottom.
The top cap is surrounded by black paint. Note the mica disc has fingers that form an interference fir to the shoulder of the glass envelope.
The classic envelope is 50 mm in diameter, and excluding the IO base pins is 123 mm tall.