The PM3 was introduced in 1926 as a general purpose triode with a μ of 14. The filament current, initially 0.1 Amp was later reduced to 0.075 Amps and the label on this specimen specifies 0.075 A filament current. Our other PM3 is an earlier example and consumes 0.1 A filament current at 4.0 V.
The identification details are printed on the dome of the envelope. looking from the base up under the silvering on the glass reveals a horizontal box anode.
This is supported from the pinch by stout wires that form a wide U. The anode is stippled.
The balloon envelope is 43 mm in diameter and, excluding the B4 base pins, is 87 mm tall.
References: 1931 data-sheet & 1004 Type PM3 was first introduced in 1926.