Big Mac is a general name like Blue Torch and doesn’t refer to a specific clone.

But if there is an original Big Mac clone this may be it. This is a Trichocereus huanucoensis that has been available for many years as Big Mac. Sold by jiimz and The Succulent Source, and previously available from World Seed Supply.

This clone was designated “P7 huanucoensis” when used for some breeding projects a while back crossing it with Skip, Awful, and others

Cuttings of this Trichocereus huanucoensis P7 Big Mac are sometimes available from Cactus Affinity.