It keeps the fire door against the wall when closed. Otherwise the draft from the fire would suck it away from the opening and let air in (fire doors aren't to stop fire, they are to stop air, so that the burning building doesn't turn into a giant chimney). The metal finger piece sort of hooks on the floor plate and into the wall when the door is closed. Because the door is hanging on top rollers only, it can swing away from the wall...
Oddly, for some reason, almost all the fire door hardware I have ever seen on old mills has patent dates or numbers on it, the folks making them seemed very concerned with that ;P
As to why it ended up in someone's odds and ends collection, could be someone just thought that what was cast into it was interesting in it's own right...(and it is)
Or perhaps they were supposed to go back and fix the door and the mill burned down before they got around to it....
PS: Your piece is , I believe, the diagonal finger shown on the bottom left of the door in the picture, improved slightly from the original patent by making the bold area wider (sensible).
Also explains the odd look of the pointed end, wear from scraping on the floor slot.
