Don't worry about it. They are left-overs from the old Assault Weapons Ban that expired on September 13, 2004. High cap magazines manufactured during the 94 -04 period were illegal and had to be marked in order to distinguish between the pre ban and post ban magazines . You'll find that on a lot of Post ban firearms and accessories.
USAF Retired, CATM, SC CWP, NH NR CWP, NRA Benefactor
To preserve liberty, it is essential that the whole body of people always possess arms, and be taught alike, especially when young, how to use them... -- Richard Henry Lee, 1787