M-Cards are issues from magazines and publications. Because many of these were often inserts in publications, most are photos/supplements and other types of inserts – not standard trading cards.
The M-Cards are different from others in that they are dominated by the aforementioned supplements. Inserting hockey cards became commonplace in the post-war era with magazines such as Sports Illustrated for Kids. But in the pre-war era, most publications chose to insert larger supplements and pictures instead.
While many of the supplements are beautiful issues, one downside is that they were often printed on thin paper. Because of that, the supplements will often be found creased and with rips or tears. It is easy to damage these fragile issues and they should be stored with a higher-than-usual amount of protection.