Here's some examples to show that the present 'earth pin at the bottom' configuration has not always been so.
This HPM power point is the first of the flush mounting post war designs, ca. 1948. This particular one is fitted to a 6VDC-240VAC shaver inverter. You can just see the BC socket beneath the GPO to accomodate shavers not fitted with the three pin plug.
Here we have a GPO from 1956,(the age of the building it came from)which is now in my garage. It's based on the American wallbox dimensions which became standard here. Most such GPO's were like the one above, but this one requires a double width wallbox as the switch and socket are quite separate items.
Here we have yet another HPM powerpoint; this one being from 1967...the house I grew up in was fitted with these throughout.
As we can see here the above was intended for horizontal or vertical mounting. 99% of the time it was horizontal with the switch in the correct position and the socket 90 degrees out. Certain things like plugpack transformers or time switches always looked awkward plugged into these and depending on the weight distribution of the internals, didn't always stay in properly.