The Ministry of Supply (MoS) was a department of the UK Government formed in 1939 to co-ordinate the supply of equipment to all three British armed forces
The Ministry of Supply was abolished in late 1959
Above from Wikipedia., so that is guaranteed to be older than I am !
If there is no evidence of filler on the castings under that scabby coat of blue paint it is likely to be "War Finish", where such nicities were dispensed with – most WF machines I have seen had a small plaque stating that they were rough for that reason.
Can't help with a maker, though I have a recollection of seeing something similar around 30 years ago at the Vickers tank factory in Leeds (formerly ROF Leeds at Crossgates). It was surplus to requirements, but there was mechanism to allow me to buy it.
It shouldn't be too difficult to come up with a replacement horizontal spindle to return it to grinding duties – a bit of plagiarism of a similar sized machine's bearing & spindle arrangment would save having to design one from scratch.
Good luck !
Nigel B.