What 1800's early 1900's cough drop posters would be worth $3400 a piece?

[1 Patriot-of-many]

My father in laws estate auctioneer finishing up the estate sales found a couple folded posters inside a book. He figured they would go for $300 a piece but they sold for $3400 a piece. I told my wife they were probably WWII posters, but found out they were late 1800's to early 1900's. Talk about crazy stuff. Any clue? We both figure her dad didn't even know they were in the book, since all the valuable papers ect were in one particular place in the house for the kids. It was rather a big deal since the big house and land in Bucks county PA went for a pittance IMO. Whoever is settling on the house and a few acres got a steal at $129,000 even though they have some fixing to do with the house.
Thanks father in law! Wish I could ask him if he knew. What's really the kicker is the kids threw out a shitload of books......