We have Door Knob Mortise antique 1873 salvage Item.  There is a slot in the mortise for a key - I do not have the keys.  The center "plunger" is ?locked as it is inside the mortise.    It is metal with a "Patented date of Oct 19 1869 & March 25 1873" on it.  It measures about 5 3/8" in H x about 7/8" across the plunger face plate x about 3 3/8" in d x 3 3/4" in h on just the black back.  There is some tarnish and light rust.

Door Mortise Antique 1873 Salvage