Every interior door of our 1890's Victorian home has similar keys.
Formerly owned a 1906 Craftsman and a 1935 Spanish Revival with same type of key.
I would suspect they were installed up to the 1940's at the latest before Schlage and others began offering alternative latching methods, like push or twist button locks on bathrooms, and the like.
Somewhere out there in cyberspace is available data and charts as to the evolution of the skeleton key.