When were shutters introduced on socket outlets in your country?

In Sweden they were introduced in the 1960:s. In the beginning the designs were akward. One design required you to slide the plug (applying equal pressure to the pins) from the top of the socket down to the holes. The shutters probably jammed the day after they were installed. (We had these in the house where I grew up. I was taught never to remove a plug from a socket, since it was almost impossible to get them back in...)

Another solution featured a turnable cover. You inserted the plug a few millimeters, turned it 45° and then inserted the plug completly.

Today sockets utilize a very simple and easy to use solution: In order to open the gates you have to push two pins simultaneously into the socket.