From Gun Traders Guide 30th Edition:
"Model 85D or 185D Bolt Action Repeating Shotgun. ... Takedown. Three-round. 20 ga. only. Two-shell detachable box magazine. 25-inch bbl., three interchangeable choke tubes (F,M,IC). Later production had 26-inch bbl. w/F/IC choke tubes. Weight: About 6.25 lbs. Plain one-piece, pistol-grip stock. . Originally designated Model 85D, changed in 1947 to Model 185D. Made from 1940-71."
"Model 185K ... Same as 185D except has variable C-Lect-Choke instead of interchangeable choke tubes. Made from 1950-63."
I would say the "D" designation differentiates from the C-Lect variable choke "K" model. It would seem logical the "M" indicates a Modified choke.
You could check
www.Havilinsales.com. Those folks operate the Mossberg collectors association. They should be able to provide any information may need.