The aircraft serial number is generally only on the main data plate. For German aircraft, many were on the side of the fuselage, and then later in the war, moved to the tail section, sometimes welded inside the frame work. This was because the tail section usually survived crashes and fires. The aircraft serial number on a part would usually have been painted on, to keep track during maintenance. Here is a part of the NASM He219 where the last 3 of the aircraft serial was painted on the part.