The 6U5G is an American Magic Eye tuning indicator 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 6U5G can be found with a UX6 base or the IO base as here. The attached data-sheet has base connections for both variants.
Printed on the base is the Brimar name and Made in England.
The display bowl with the amplifier triode below.
The wide glass tube envelope is 28 mm in diameter and, excluding the IO base pins, is 86 mm tall.
References: Data-sheet, 1043 & 1040. Type 6U5G was first introduced in 1937.