In my experience, most square tank Evinrudes start with serial number 00001 with each different model number. The serial numbers of my 1936 to 1941 Sportfours range from 00028 to 01070. Like most or all 4 cylinder opposed Evinrude motors, the Sportfour muffler has 8 bolts holding it onto the cylinders, (2 of which are hidden bolts, located on the front of the motor, between the top and bottom cylinders). Don’t force the muffler off without making sure all bolts are removed first. My ’37 Sportfour is one of my favorites. It is easy to start and very smooth and quiet. Use the recommended oil ratio – no less. The factory ratio is One Pint per gallon of gas for general use. For break-in or racing it is One and a Half Pints of oil per gallon of gas.