Spotting a boxed slate stone at a car boot, I bought it (£1.50). The seller
said that it was part of his g-father's tools, which he was also selling;
all woodworking tools, saws, chisels, gauges etc.

I took the stone home, noted that it was nearly flat (unusual), and that the box
wasn't saturated in oil/grease (which it normally would be).

I splashed on some water, which beaded up, indicating that
the stone has seen at least "some" oil.

I lubricated with white spirit ("mineral spirits" in the USA)
and tried my normal "trial chisel" (an old, pitted pre
1920 Sheffield model). The stone cut well, forming black "ink"
in the white spirit, gave good grip/feedback,
and left a near mirror finish.

The box is labelled Silkstone/Cambrock, and google bought me here.
I can add little to what has already been said, but I can say that
as a woodworker, this stone would make an excellent partner
to follow a Norton India fine - India fines abrade quickly,
but they're not quite fine enough to give a good cutting
edge, but the silkstone is fast enough for the grit
jump not to be excessive, but fine enough that the resulting
is usable for all but the most critical smooth planing.

I wonder which quarry is came from?

My next stop (given my other hobby of family/social history
research) is the British Newspaper Archive, which I have
a subscription for, to see if I can find any information.

I won't bother posting photos, since my stone and box appears
identical to the photos already posted by others (which implies
these were sold in some numbers)

BugBear