Interesting place the proofhouse. There is a story and some pictures on this link; hope you don't mind a link to another outdoor site.
http://www.modernsportsman.com and scroll to second article on homepage.
I go down there quite a bit. They have a very interesting museum there and do accept visits by arrangement usually catering for parties of 10 or so. Very litle has changed since it started back in 1813. Their Archivist, Bill Harding, wrote an interesting book on Eley Cartridges and is just in the process of completing one on Kynoch and other Birmingham ammunition makers. Worth a visit if you ever cross the pond. It was probably Bill who sorted out your query. Lagopus.....
