Gentlemen; I'm sure many of you are familiar with the letter
by Richard Knight of Foster Lane, describing
29 sharpening stones, including the oft-sought after
"13. Devonshire oil-stone" and "14. Cutler’s green hone".

Here's one of many copies on the web;

The Principal Stones Used in the Mechanical Arts | Lost Art Press

Reading the heading once again, I noticed
that the helpful descriptive letter was actually
a catalogue of a physical set of stones, presented to the society.

"The Society Instituted at London for the Encouragement of Arts, Manufactures, and Commerce"

This is now called the RSA, and is still going.

https://www.thersa.org/

So I emailed them, asking if the 29 stones could still be found in their archives. This
(of course) would be stunning - actual examples of Devonshire oil stone and Cutlers green,
that could be examined by geologists.

I received a surprisingly quick reply from their archivist (2 days turnround).

"Thank you for your interest in information from the RSA archive. Sadly we do not have any of the stones listed in the article and no original letters etc. from Richard Knight to the Society."

It was worth a try.

BugBear