Yes, I saw Crabtree moulded into the porcelain bit of the socket.
The 15A rubber plug didn't come with the other bits, I just used it for scale in the picture. There was a 15A plug with a missing pin though
Re theatre lighting, ISTR that "Grelco" is used in that area as a generic term for two way adaptor (like "Hoover" in domestic parlance for vacuum cleaner). At least it was when I was at uni.
Off topic anecdote:
I heard that a present day techie in said dramatic society was heard to wonder why the small storeroom next to the tech workshop was known as the "choke" room. I knew- it originally (and still did when I was there) contained the saturable reactors (chokes) which were the main dimming elements of the state of the art (in 1956 or thereabouts) stage lighting control system. I replaced a few dead selenium rectifiers in the dc saturation current drives with 1N4007s during my techie tenure in the '70s.