Very Old Trigger Mechanism

this brass and iron device looks very old and may be rare. Curved brass arm at the top is spring loaded and very powerful. When lifted open it can be place under the trigger at the rear where brass nob is on the back of it. The trigger at the rear has three holes to choose from to tie possibly a trip string to. When activated, the brass arm slams down onto a slot at the front presumably holding another string which would then pull the trigger of a weapon. There are two holes in the base for mounting it to some sort of post or base. I could not find any writing on the tool at all. It’s been suggested that is for picking up and dropping off bags of mail at the train station. But I don’t think so. It’s too sensitive and complicated for that. It should be an interesting topic for discussion.