They're all the plates that start collecting in the cub board. Each engraved with a certain image pertaining to the accomplishment. There isn't a individul momento fo the headless horseman though. But there are ones for completing the challenges,exploring the areas, fortress cleared, etc.
weird. cause there are the banners for each of the hunts, so i'd imagine something like it for the frontiersman challenges, especially since it says "memento unlocked in manor" after every sub mission.
perhaps they are supposed to show up but are glitched right now?