We attempted to letterbox that afternoon. Attempted being the important word.
Unlike geocaching, the letterboxing site doesn’t have a way to log in and say you found the box, you didn’t find the box, the box was damaged, etc. so there’s no way to know if the original box even still exists. We figure it was either hidden well enough that it got overgrown and truly buried or it had been taken by park goers before we got there. Anyway, it was fun to try.