According to the book "Uniform Buttons Of The United States 1776-1865" (by Warren K. Tice), your Massachusetts Artillery button dates from after 1865. The book says a December 1865 Regulation from the Massachusetts State Militia ordered that the branch-of-service letter (in your case, A for Artillery) be added to the Militia's uniform buttons. It was not present on the buttons prior to that Regulation.
Therefore, I think it's most likely that your postwar-timeperiod Massachusetts button was lost by a yankee soldier during the Reconstruction/Occupation era in North Carolina.
Your button's front is shown in the Tice book on page 324, and is listed as button MS300A2 with backmark saying Treble Gilt. In the Albert button-book, it is MS-38, but the Treble Gilt backmark is not listed.