Third Buckle/Strap

[MacMillan of Rathdown]

That makes perfect sense to me, Matt. The ease of alteration sounds like a good reason for a kiltmaker to prefer that method.

Is it fair to say, though, based on the history of it, that it's purely an American innovation to the kilt?

I rather doubt that the third strap is some form of Americanization of the kilt; All of my kilts were made for me in Scotland, some nearly fifty years ago; The oldest kilt still in my tin box is probably forty years old, has three buckles, and was made by-- wait, I'll have to go look-- Chalmers of Oban.