The 12U5G is an American Magic Eye tuning indicator design and its construction follows the normal pattern. The electrode assembly is mounted centrally in the envelope and the display is viewed through the dome.
The vertical stack: pinch, triode & display.
The amplifier triode with the anode connected to the deflector.
The display bowl.
The wide glass tube envelope is 28 mm in diameter and, excluding the UX6 base pins, is 88 mm tall.
References: Data-sheet, 1043 & 1040.