M-Cards are issues from magazines and publications. Because many of these were often inserts in publications, some are photos/supplements and other types of inserts – not standard trading cards.
Boxing was sort of the exception to that, however. There are several publications issues in boxing that were standard trading cards. Several of these were printed in the early 1920s and feature some of the first cards of a few key boxers, including the legendary Jack Dempsey. While a few were cataloged by Jefferson Burdick in the American Card Catalog, most were not.
Dempsey’s 1922 cards produced by Comic Life and Boys’ Magazine are some of his first cards. A few collectors even consider these to be rookies of a sort, even though Dempsey does have a few earlier cards.
In terms of the supplements, note that those inserts that are not cataloged in the American Card Catalog (with M- designations) have not been given individual pages below. However, a list of those sets can be found here, on a separate page.