I am actually beginning to believe it IS a yamaha.
the 684 code 684C, the c according to several mariner databases denotes 1991 year, the 684 is a code for a yamaha 15B also.
Checked into pistons..
If you compare this piston, for a yam 15C, apx 1990, to a mariner with the 684 code, they are identical. also, the head gasket is identical.
the pistons are dirt cheap, and the full gasket kit is too.
Also the crank bearings.
It would certainly appear to be a 1991 yam 15.