It's interesting that the 2-round pins had been settled upon by 1900!
Clearly it's an ancestor, or very old cousin of the modern CEE 7 family (i.e. Schuko & the French system), as well as BS546 (and BS1363), the Italian, Swiss and Danish systems.
I wonder where the notion of two round pins originated, as it seems to crop up all over Europe.
Modern French grounded, recessed and shuttered socket, and grounded CEE 7/7 plug... still looks VERY similar, even if it's now a lot safer.