I think the gas caps for the Evinrude Sportwin, Fastwin, Fleetwin, and Speeditwin all had the same size thread from ’23 to ’29. Caps after ’27 had a vent screw, since the tanks up until 27 were vented internally. (That’s what that little pip on the Speeditwin T and TS tanks are for.) THese caps should have a steel oil cup integral with the cap, to measure oil into the tank when you fill it. Did yours have the oil cup? I have never seen one of these with a safety chain. I have, however, seen a lot of them with the oil cup rusted away, leaving them to look like a more conventional cap that would need one.
Tom