I believe it's just U. americana. I don't believe there is a specific cultivar attached to it. As far as I know it wasn't planted since Algonquin Park isn't known for it's planting program and if something were to be planted it would have been native stock.
Maybe I'm wrong and some tourist did some guerrilla planting 30-40 years ago? It's not likely, but could explain why the tree has the dense balloon shaped form.
Next time I'm in the area I'm going to do a little more in depth investigating.
You got it Tom.