An internet search for Massachusetts Service Medal came up with the following from the Massachusetts General Laws, Part 1, Chapter 5, Chapter 33 (Militia), Section 67. Not sure when it dates to but I thought the use of the Great Seal as a military emblem post dates 1900. The last part may refer to this medal with the Great Seal attached.
"Section 67. (a) Each member, who completes 3 years of honorable service in the armed forces of the commonwealth, except the inactive national guard, shall be awarded a service medal and for each additional 3 years of like service a clasp to be affixed to the ribbon pendant thereof. All other members of the organized militia may be awarded a service medal for completion of 3 years of honorable service and for each additional 3 years of like service a device to be affixed to the ribbon pendant thereof. Members of the armed forces of the commonwealth, active, retired or honorably discharged, who have served in the armed forces of the United States in time of war and have been honorably discharged shall receive a device indicative of such service, to be affixed to the ribbon pendant of the medal herein provided."