I'm assuming Grandad was an American and/or served in the U. S. Armed Forces. 533d Engineer (Boat & Shore) Regiment served in the Pacific but that doesn't seem to fit with later service with a "commando unit".
Could he have been some sort of engineer (flight, etc.) with Army Air Forces? 533d Bombardment Squadron (Heavy), an element 381st Bombardment Group (Heavy), saw duty with Eighth Air Force in Europe from June 1943 to April 1944. 533d Fighter Squadron, an element of 83th Fighter Group, was assigned to First Air Force and trained replacement pilots during the war. There were also three air commando groups which were elements of Army Air Forces.