The 1U5 is a diode pentode designed for detection and first audio voltage amplification in light weight portable radio equipment and the sensitivity of the pentode allows for a low anode voltage thus saving weight on the HT battery.
above the top mica is the diode anode and the filament tension spring is hidden by the gettering. The filament is a single coated inverted V. The diode anode connection passes inside the main anode cylinder.
The pentode section with three wire grids.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 47 mm tall.
References: Data-sheet & 1040. Type 1U5 was first introduced in 1945.