Looks like 1/4-20 maybe 5/16. Haven’t used that paper in years. No on rocker cover bolt but good guess just not on those valve covers. What size is the head?

With the blue paint down the shaft, looks like an alternator bolt to me, that holds the two halves together

Bolt from the raw water pump?

Maybe.
Not the type ) a 1994 ABC Merc but not HP500 I pulled apart this week-end though to redo the brushes.

Possibly mounting clamp bolt for the oil cooler?
Just throwing out there with the intermittent paint coverage down threads and coated bolt....???

At least something that needs ease of self centering the way it's canted/tapered on the end. Possibly threads into some type of clamp. Doubt oil cooler but maybe on right track.

After seeing that alternator, I'm so glad I play in fresh water. That looks like it was used on salty roads in the winter

So, who has the closest guess ? I would guess the combo hex head with phillips would help alot, lol.

Lake Winni, NH for 26 yrs. 4 months in west coast Fl about 22-23 yrs ago.On lift. No other corrosion anywhere, even outdrive. Unprotected aluminum gets chalky for sure.

Does the closest guess get a cookie?
I like cookies mmmmmmmmmmmmmm

See ya Later has left the building