I wonder what the intended use for that 3-prong plug was! It can't really have been earthing if there was an Edison adaptor. Maybe it was designed for two hots, e.g. from a 3-way Edison socket.
Edison to socket adaptors were banned in Germany rather early, actually in 1959 (along with plug-in multi adaptors for Schuko plugs as they were considered dangerous for the mechanical integrity of the socket). Don't know when they were banned in Austria, but probably not much later. Last time I've seen one in use was back in the 90s and they were rare by then.
