I have spent a lot of time with various databases, with Google Scholar, and with the internet in general, and have yet to see any source. I'm relatively new at this database thing so it's completely possible I've missed some button or link.
My question above was if you could identify a source saying why the change elevating var. truncata to a subspecies (effectively, to the exclusion of the other varieties, including the type). That is not something you will see in any of the standard references. Without that information I am going to assume that the type population deserves to be in the type subspecies. These plants have been grouped for decades (see my comment below). I would love to see where they were broken up/ordered this weird way, because it would help me figure out the reasoning behind it.
Artichoke Agave (Agave parryi var. truncata)
