The National Arms Co. marked guns were made for H & D Folsom Arms Company of Washington, D.C. Marlin did indeed make guns for this company and several different models during production for them. Usually, private brand guns were not marked with the makers name, just the distributors at the time.
The "M" in the barrel serial number indicates your barrel has a modified choke.
As to the 1926 manufacture date, that would be hard to say without knowing which model of shotgun you have. A picture would help. Or, at best, a complete description, i.e., is it a takedown action? Is it a straight grip stock or pistol grip? Is the "hang fire" safety release a small button up near the hammer, or a lever, down by the trigger?
Looking forward to your reply........
Due to the increasing cost of ammunition, there be no warning shot!
The growing federal deficit = generational slavery to the national debt.