The PM4DX is a medium impedance triode for use as a detector. Exhibit PM4DX has the identification etched into the glass, this exhibit carries a paper band on the Bakelite base and has details printed on the dome of the envelope.
The balloon envelope is 43 mm in diameter, and excluding the B4 base pins, is 87 mm tall.
References: 1931 data-sheet & 1043. Type PM4DX was first introduced in 1928.