I've seen that on plenty of old appliances in Ireland. In fact, I think we have an old Nilfisk Vacuum cleaner that has one.
It was definitely common on things like hot plates, grills, sandwich toasters etc.
I have never seen the version above, with the external earth cable though. The examples I have seen here, had a normal internal earth and two scraping contacts on the edge of the socket. The 'plug' was recessed in the appliance, or shrouded. Similar kind of set up to schuko, just smaller.
And also, an old (1970s) vacuum with two blade pins at arranged at a 45 degree angle i.e. / / but in the same configuration as above.
I have also seen appliances with what looks very much like a "Europlug" CEE 7/16 fitting.
The IEC 'kettle lead' that we find on the back of PCs etc seems to have replaced all of these fittings.
Interestingly, I had a Bosch fridge-freezer which had an IEC plug on the back! It was contained in a connection box on the back. To remove the plug you had to open the screws and open a door on the box.
I can only assume they must have done it this way to allow for easy localisation with the correct cables for 'weird countries'
That's the old UK kettle lead (Also used here) :
Oddly enough, it seems to mate with CEE 7/7 and CEE 7/16 plugs! It's weird how often that same pin configuration got used!
The live and neutral terminals are the two round holes, and the earth is the rectangular hole at the bottom. The other round hole is just a screw for holding the thing together.
These seem to have disappeared by the late 1970s and were very much associated with non-automatic kettles i.e. the type that would just boil until they boiled dry! It was common enough to walk into your kitchen only to find the entire room filled with steam and the wall paper peeling off !!!
I think they may have had some kind of safety cut out, but they certainly didn't seem to have the usual auto-switch off once boiled.