I have a friend that works at the DU Headquarters in Memphis TN. And since I did not know the answer to your question even being a DU Member my self, I contacted him. Here is the answer:
The 10-30 notation on the receiver was an incentive program hosted by DU for the DU Chapters. To make a long story short, the chapter must have their paperwork turned in at the ten day mark and the thirty day mark after an event in order to qualify for the free 10-30 gun. If the chapter qualified for the firearm, Du's national headquarters donates a 10-30 marked gun to the chapter at no cost to the chapter for their fundraising effort at the event. DU frequently used Mossberg for these programs. Enjoy your new shotgun!