I once saw an auction on eBay about 10 years ago where a seller had a sealed case of FF 48 books inside, i can't recall if the box contained other FF issues. He said that he bought it in some gas station either back in the 60's or 70's. I think he bought a couple more of those boxes, one of which he opened. I dunno if anyone else remembered seeing that auction, but it was unique and amazing to see.
Maybe that may have affected the quantity of FF 48 in high grade if the buyer opened it up and sent to CGC.