If the caliber is not stamped on the barrel, I would think that it is a factory oversight...I'm not sure it ads to the value, but a serious Marlin collector might be interested in it as a "curio". Also, you mght check with Marlin, they would probably like to have it back to stamp caliber on it ( liability issue ). For that matter, if you haven't shot it yet, you mght want to have a gunsmith do a chamber casting, to insure it has been properly chamber, and is indeed a .44 mag.
Mark