All Stihl saws have a series number. The first four numbers of any parts unique to that saw will start with that series number. Look at the bottom of the tank/handle housing, and see if you can find a 11 digit part number. If it starts with 1121 it is a 024/026; 1125 is a 034/036; 1118 ia a 028; 1119 is a 038.
Sometimes a part from an older series will be used on a newer saw, but usually not a tank or cylinder shroud. The part number on the part may not always be the replacement part number for that part, however. It is best to use an IPL to be sure.