The show exaggerates many things and entertains too many wild theories but I don't think the Lagina's allow the show to be completely fraudulent.
My take on the stone is this. The Oak Island Treasure Company had a stone in the book binder's window while they were trying to raise funds. The stone was described by Harry W. Marshall as "..some lad had put his initials "J.M." on one corner, but apart from this there was no evidence of any inscription either cut or printed on the stone.".
It is very possible that Marshall wrote this years later and thought he recalled the initials as J.M. but they were actually L.N. It wasn't an affidavit, just a letter. If the show just created this stone as a fake, they would have put J.M. on it as that is the only written description of the stone.
Obviously, it is preposterous that the Lagina's went back to the building and happened to find the stone in the basement. Rumor has it that they acquired the stone from a private owner. For whatever reason, Prometheus didn't think this was exciting enough so staged "finding" the stone where it was last publicly seen. You can tell this because the guys on the show aren't actors and that was as unnatural a scene as could be.
So I think it's reasonable to assume that the stone they "found" is in fact, the "90 foot stone" that was at A&H Creighton. However, that stone was likely not the "real" 90 ft. stone and instead, just something used to help raise funds. In all probability, there never actual was a stone found in the money pit.
