Invented 99 years ago and we're still trying to get rid of them.
Reason I say that, guys, is because every now and then, I'll go to a house over here, where a guy has one of the Bayonet Cap versions of these adaptors, in his garage and he's got it wired up to a 2 bar (2000W) heater and wonders WHY he keeps blowing lighting fuses.
This sort of thing was banned over here back in 1976, but some people have still not heard that news-flash.
Let's face it, these days if you're not young, you're old - Red Green
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.