The odds of finding a copy would be slim, maybe none, now, but back in the early 80s there was a bookstand pub edited by Jack Lewis, titled "Gun Digest Black Powder" or something along those lines. It devoted one chapter each to extolling the virtues (both real and imagined) of Dixie Gunworks, CVA, H&A Underhammers, and the Mowrey Boxlock.
As I recall, the tone of the writing wasn't very scholarly - in fact, more along the lines of a writer trying to sell his advertisers' repro Italian guns. Still, there were some words on the history of the gun (I think it was referred to as an 'Ethan Allen rifle', perhaps after the original 19th century maker), discussed the pros and cons of the lock deisgn (pretty short discussion since I think the lock was a sear, a tumbler, and a spring), and a bit about the then current maker and the design's reintroduction.
A google search on "Jack Lewis Mowrey Gun Digest" should turn up some used copies.
