The MHL4 was used in professional and military applications over many years until it was superseded by the L63, which had comparable characteristics.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 an earlier example MHL4 with a balloon envelope.
The osram paper label.
Through the top one can see the three insulated heater loops and the grid supports.
looking up from the base the pinch can be seen and above the rectangular lower mica. The anode is a plain grey rectangle.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 95 mm tall.
References: Datasheet, 1040 & 1043. Type MHL4 was first introduced in 1929.