I have that same edition of the Gun Digest Treasury. The series of articles by Kennett update Baron Engelhardt's work. (For example, Engelhardt's article on British proof ends with the rules of 1925. Kennett's includes the changes made in 1954, when the Brits started using the "tons" marks.) The two of them together are likely the best and most complete source on proof in most foreign countries, although as noted above they also are in need of updating.