The MHL4 was used in professional and military applications over many years until it was superseded by the L63, which had comparable characteristics. We have a later example MHL4 but the balloon envelope version came first and was made with the B5 base and a B4 plus side terminal base cap.
The identification is etched into the dome of the envelope. The later version has the M-OV lozenge on the side for the identification information.
The anode is mesh, the control grid is held on two sides and wound so as to pass flat across the coated cathode tube. The oxide coated cathode connects at the top and the heater connects at the base of the cathode tube. Mica supports hold the electrodes in place.
A closer view of the top mica and electrodes.
The number of spot welded components around the top mica indicates an intricate manual assembly.
The balloon envelope is 49 mm in diameter and, excluding the B5 base pins, is 112 mm tall.
References: Data-sheet, 1040 & 1043. Type MHL4 was first introduced in 1929.