Platoon Sergeant Major was a rank introduced around 1939 and only last to 1941/2 IIRC. The purpose of the rank was to promote senior and experienced NCO's to a rank where they could command a Platoon or equivalent in battle, thus replacing the need for an officer. It was a only a short term measure to help reduce the need for junior officers due to a shortfall of officers at the start of the war. The badge they wore is still used today in the British Army- The Sergeant Major's crown surrounded by a wreath, the same used by RQMS's today.