To me that looks like the rubber cover I would see on the Evinrude and Johnson 3hps to cover the nut that holds the prop on(was probably on some other motors as well). Although flattened and it looks like the would have cut the top point off if that’s what it is, which I did to fit a Caille(Brass bladed prop 2nd picture, 1st picture is a 1953 Evinrude 3hp, it was also on my 52 Evinrude 3hp and my 1966 Johnson 3hp).