The Y63 is a second generation tuning indicator with a 6.3 Volt heater. This type of indicator was used in many radios. The data-sheet gives that the 6G5 is electrically identical to the Y63 but has a UX6 base cap.
The Osram label for this M-OV valve. No other identification survives.
The triode with the uncoated cathode tube passing into the bowl where the cathode is coated again.
The display will glow green when in use with the shadow changing size according to the input voltage.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 93 mm tall.
References: Data-sheet & 1040. Type Y63 was first introduced in 1937.