Mmmm, the thick plottens.

This is the link to click on gun research, but doesn’t work any more! Scroll to the bottom of the page.

https://www.hollandandholland.com/gun-room/gun-room-services/gun-repairs-renovation/

Maybe you could contact Donald Dallas, he has written books an H&H and other gunmakers etc., and good books they are, or perhaps online to Diggery Hadoke etc. They would know what is going on. It would be a shame if we have lost this historical resource.