I sent the photos to a few tool collectors and one of them replied with the answer. Turns out that it's a key for a watchman's clock. When the watchman was making his rounds, he would use these keys that were found at the watchman's stations, and insert them into his watchclock, which would make a record of the time he was at each station, proving that he actually kept watch. Once I knew what it was I was able to find the patent for it:
If I look inside the slot of this key, I can see the number 16 on each side, one convex and one concave, see number 35 and 36 in Fig. 1 above. These numbers would leave an impression on a record disk (number 33 in Fig. 3) in the watchclock.
