The Mazda 6M1 tuning indicator was a replacement type and not current in the mid 1950s.
The construction follows the usual pattern with the triode like amplifier above the pinch and the display viewed through an end window. The screen is circular and the central electrode is a small black button. The full display range is given for a 22.5 Volt alteration in grid voltage.
The central triode voltage amplifier connected to the display at the top.
The common cathode and conical bowl.
The display end.
The wide glass envelope is 28 mm in diameter and, excluding the IO base pins, is 70 mm tall.
Reference: 1040. Type 6M1 was first introduced in 1937.