George,
Douglas used a 7/32-24 on a few things like the flat head countersunk screws holding the rear plate onto the EW clutch and some suspended 2-3/4hp fuel tanks that used a countersunk, dome head screw. But they kept with the 3/16-27 thread for the timing cover long past the EW era.
It sounds like your timing cover may have been re-tapped for 2-3/4hp petrol tank screws.
-Doug