I just learned that this is a watchman's key dating to the 1880's. A watchman would find the key chained to a station along their tour. They would insert the key into their watchclock, where it would cause a paper disc to be stamped with the station number, date and time. The piece extending from the side caused the marking. The number on the key would correspond to the station to which it was assigned.
Thanks all for your replies.