"they had been together again for the Allied landing"
Troops Crouch Down on Sword Beach as they Wait the Signal to Advance
Public DomainTroops Crouch Down on Sword Beach as they Wait the Signal to Advance - Credit: Mapham J (Sgt), No 5 Army Film & Photographic Unit

Stevens is talking about Operation Overlord, the D-Day Allied Landings on the coast of Normandy on 6 June 1944, which marked the beginning of the end of World War 2. Five beaches were chosen for the landings: American forces would land on Utah and Omaha, the Canadians on Juno, and the British on Gold and Sword.

The batman uses the term "posh geezer" which makes him a southerner and probably a Londoner. It's therefore more likely that he was attached to the British 3rd Infantry Division landing on Sword rather than the 50th Northumbrian Division on Gold.