You need to keep in mind that none of the general reference works that list gunmakers are wholely reliable. This is especially true of Gardner as the first edition of his book came out in 1936 or 1938, the last around 1968. This is true for virtually all the others as well, including Stockel and Sellers. All have good information and all have errors. Gardner could easily have been confused by several versions of the same name. Nearly all of the compilers of such works used dates based on their (or someone else's) guess based on the appearance of actual guns. As we learn more, and rely more on primary documentation, many of these errors become clear but, of course, Colonel Gardner died many years ago and never had the opportunity to update his work in the light of new scholarship.
There is an excellent French language catalog of French gunmakers though, as I am out of the country, I cannot put my hands on my copy. Even lists based strictly on directory listings - like Bailey & Nie's excellent work on provincial English makers, has shortcomings as not every maker placed directory listings and some failed to update them even when they were placed.
This is a much more complicated area of study than is commonly believed.
