[emeritus]

Foyles book shop on London was still had some 2 pin 15A plugs in use in the 1970's.

I think the last vestige of the use of "Magnet" in GEC was their travel department, which was still called "Magnet travel" into the era when they relaunched as Marconi and went down the plughole.

"Magnet" is not used in GEC's catalogue for 1892, but is one of various trade marks used for a range of products in their 1910-11 catalogue. It is not used in the section on mains plugs & sockets (other than on an illustration of a "factory" connector), or in the text of the lampholders section, but can be seen in some of the drawings, stamped on the outside of some of their brass BC holders and embossed in the switch dolly of switched lamp holders of the type where the switch is turned on and off in the manner of a gas tap. I think the OP's plug must be from the inter-war period. The designs of the plugs in the 1910 catalogue are very different. (I did post this extract a few years ago but I guess it can stand another airing.)