I seem to have inherited a spent 20mm cartridge case, allegedly brought home from France in '18 by my stepfather's father, who was a (non-flying) officer in the RFC from 1917. He was at home for WW2, as he was 53 when it started, so if this is a WW2 artefact it must have some other provenance. As far as I know - ARRSErs correct me please - the RFC never had anything heavier than the .303 Lewis gun in their aircraft and I have always assumed that 20mm only came in with Oerlikons at sea, and Hispano-Suiza aircraft cannon in fighters later in the WW2. Base markings are as follows: Rim of cartridge: 8, RL, DD1 3.468.B Base of primer: 333, W8|28 with an M or W next to it, S and a theta in a square box, No 5 II N, a broad arrow in a square box, and a broad arrow on its own. Here's hoping this means something to ARRSE ammo buffs.