The Z8216 looks like a PX4 and an internet reference says the Z8216 is equivalent to the PX4. However the PX4 has a 4.0 V filament and takes 1.0 Amp. The Z8216 consumes one Amp at four Volts but does not heat-up. Increase the voltage to 6.3 V and the filament reaches operating temperature, the current is then 1.4 A.
The etched information is typical M-OV but no brand name exists on the envelope.
The bright anode is wide but has a small front to back dimension. Eight filament strands pass through the valve from top to bottom.
The top mica is held in place by the envelope shoulder.
The top mica and above it is a small mica to hold the filament tension springs.
The pinch and lower electrodes. The filament is wired from the centre and the two outer strands are strapped together, effectively forming two parallel filaments.
The classic envelope is 65 mm in diameter, and excluding the B4 base pins is 120 mm tall.