The acorn valves were designed to have low capacitances and therefore work to higher frequencies than the normal valves. The DS311 was designed for the low UHF region at about 300 MHz. The 12.6 Volt heater would be for vehicle or mains use. For portable use the DS310 had a 2.0 Volt heater.
The Wehrmacht approval station BAL 560 with the Lorenz logo on the right.
The small triode section held in place with mica strips. The insulated heater is wound around a central ceramic core.
The acorn envelope is 15 mm in diameter and, including the base pins, is 41 mm tall.
Reference: Datasheet. Type DS311 was first introduced in 1938.