Re: 0303714 | SCREW OMC, Johnson & Evinrude
You talking about the one that holds the inner shift lever to the shifter shaft? I don't have one handy to double check, but I THINK it is 10-32. One thing for sure, it is a hex head and uses a tab washer to keep it from loosening. Stainless steel, of course. I'd say you won't find one at the corner store.
EDIT: Yep, just checked that is part number 303714 as you stated. Tab washer is 303715