Technically, the version with the shoe keel is it’s own model, not called “standard”. Besides the shoe keel, it also is a more rugged boat overall with 4 additional ribs compared to the model with the standard keel. I have never heard of the shoe keel being used on any model except the heavy-duty 17-footer.
I have only seen the shoe keels on the 17 foot tandems with extra ribs. I have been told that this model originated as a result of a special order request by some early whitewater boaters in Tennessee, a couple of whom I know or knew. This was in the pre-Blue Hole era when the only alternatives for whitewater tandem canoes were fiberglass and wood and canvas. Once Royalex became available, there would have been relatively little demand for this type of modification, so I doubt Grumman would have offered it in other models.