Steve,
Through the years, Gun Digest has run at least two series discussing proof laws in various countries, including the ones we are interested in. One of these series was in the 1950s and another was in the 1970s. You could refer to these for information( it should be noted that these are mostly the author's explanation of each nations laws, as stated in the laws, themselves.).At one time, I believe Dietrich Apel, founder of the German Gun Collectors Assn., was putting together a compilation of all these articles. This would require a considerable amount of research work to locate the authors and obtain their permission.
With regard to the different mechanics, their marks are not proof marks and were not lawfully required. Some of these marks are generally accepted marks that Id the barrel maker, or in some cases the maker of the gun that is not otherwise marked(ie so called guild guns).I think some of these can be found on Dietrich Apel's other website( not GGCA). Other marks, especially on German and Belgian guns are the personal marks, to Id work done by individual workmen, on a "piecework" basis, to both allow payment for the work, and set responsibility for it. I believe Raimey has authored or is authoring a/some publication(s), including many of these marks. He might be encouraged to chime in.
Mike
