The VT50 is a service designation for the HL2K, designed as a detector or audio voltage amplifier triode in battery receivers.
The bright anode with ribs for rigidity.
Side view of the anode, pinch and pinch stem. The actual triode is small compared with the envelope.
The tension springs close-up. The top of the spring folds over and descends through the centre of the spring and ends in a hook. The feint lettering is all on the dome.
A closer view.
The wide glass tube envelope is 29 mm in diameter and, excluding the B4 base pins, is 72 mm tall.
References: Data-sheet, private communication, 1040, 1043, 3002 & 4001. Type VT50 was first introduced in 1934. See also1934 adverts.