Hi,
I've got one of those MK plugs with the spring loaded shields, so maybe not that rare, although yours is in better shape than mine!
Our council house was built in 1953 and had the earlier type with a screw for the live terminal (On the left of the photo). So, at least, early fifties. The later plugs had a nut and washer for the live terminal which made them easier to wire, but also made the body a little deeper.
When the red pointer under the cover screw was discontinued, the moulds were machined out which left the later plugs with a "bar" running across the centre as shown on the right of the photo.
The next photo shows a "Clix" stackable 13amp plug, and finally my favourite MK plug/socket combination (In dire need of a good clean and polish)
I really must get out more!
Cheers, Pete