The ME41 is a magic eye tuning indicator. It has a 4.0 Volt heater and a British Octal base. The Mazda version of the octal base was not adopted by other manufacturers and is now known as the MO or Mazda Octal.
The target ratings are listed below as screen ratings.
The triode voltage amplifier section with the indicator on top.
The triode section end on. The indicator bowl is circular.
The end window envelope is 28 mm in diameter, and excluding the MO base pins is 74 mm tall.
References: Datasheet, 1040 & 1043. Type ME41 was first introduced in 1939.