I recently bought a used Merc 90 from Stevens Marine.
It too has the side fins on the foot, but it has something I have'nt seen before.
On the rear, lowest part of the foot, at the base of the grates, it has a sharpened plate, about 1" wide, going side to side, angled down to catch more water.
I have'nt not gotten out on the river yet to check it out, but it makes perfectly good sense to me.
The guy who owned it before me must of known something about jets, because he had put in a polished, 4 blade stainless impeller.
Has anyone else seen this on a foot before?