If your grandfather painted the motor like a “camo” green (Johnson had a green like that in those years too), I think I see a light blue underneath that would indicate Evinrude.
1969-70 Evinrude 25 HP was darker blue and had a different style bottom pan to accommodate a different style hood.
Around ’74 they went with externally mounted coils on the cylinder head because of a different ignition system and yours doesn’t show that.
Which would narrow the years down to ’71-’73 Evinrude.
I think the last year for pin drive prop was around ’77 before going to spline drive.
I don’t have a serial number chart for the 70s so I can’t determine anything from the number on the freeze plug. The “M” I think stands for “Marine” corporation (I could be wrong) but OMC used that to identify their OEM parts. I don’t now how far back they started doing that.
Pictures of the whole motor? Cover/hood design, color, decals can help determine the year too if those aren’t painted over. Even the intake/bypass covers and lockout lever assembly might give clues.
