Lagopus, I think you are describing Nigel Brown’s 3 Volume work, British Gunmakers.
The single volume Douglas Tate book is excellent on Birmingham gunmakers. I was very lucky and treated myself to a remaindered copy at the Game Fair some years ago for a fraction of the price now being asked for second hand copies (which in itself speaks for the desirability of the book.
One of the best guides for a purchaser is Diggory Haydoke’s British Boxlocks.
You need to remember that the Birmingham makers each produced guns in various grades, and whilst the lowest grades were almost always sound purchases at that price point they will not compare with the same makers better offerings .
