I just bought an old catalogue on Ebay, dates ca.1912
Inside there is a picture of almost the same plug.
They call it in German: Steckdosen aus Fayence (eng. Modelle).
There is also an English translation of this item and then they call it Wallplugs (English types).
Fayence is the material, a sort of pottery.
A friend of mine has almost the same as yours including the outlet, but then colored, red-brown, with gold lines.
I think it is made for DC power, because there is only one way to make contact with the outlet.
So I think this plug of yours must by very early, beginning 1900/1910.
A very nice item.
I have a strange plug myself, made out of wood with some brass components.
6 Amp, 250 Volt
But it is the opposite of your plug
It hasn't a pin but a hole.
If you like I will make a photo of it and put it on this forum.
Wieger Nieuwenhout, the Netherlands