The AF100 is a German military valve from 1940. The pentode has an anode dissipation of 4 Watts. The central base pin connects to the external metallising.
The reverse carries the Wehrmacht stamp and the BAL acceptance office.
The problem with acorn valves was that inserting them into the socket could, and often would, crack the glass and thus ruin the vacuum. The German solution to these high frequency all glass valves was to use wires from the glass seal and to have an external Bakelite skirt that held the pins.
The wide glass tube envelope is 26 mm in diameter and, excluding the base pins, is 64 mm tall.
Reference: Data-sheet. Type AF100 was first introduced in 1940.