The 1R5 (DK91) is a miniature B7G dry battery heptode, it was designed for frequency changer use in battery portable radio receivers for medium and long wave. The valve was designed for self oscillating service.
This exhibit was imported to the UK and is marked as Foreign made.
At the top the wire grids and filament tension spring are clearly seen.
The octagon etched into the envelope has the Type as 1R5 - the American designation.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Data-sheet & 1040. Type 1R5 was first introduced in 1940. See also1940 adverts.