According to the book "American Military Button makers and Dealers; Their Backmarks & Dates" ...that particular form of Horstmann backmark dates from 1893 to 1947.
Since you did not mention looking for your MG button in the Albert button-book... let me suggest purchasing a copy of "Record of American Uniform and Historical Buttons" by Alphaeus H. Albert.
Your button is shown in a photo in Albert's button-book, on page 195. It is the emblem of the Morris Guard, of Atlantic City, New Jersey, a state militia unit first organized in 1887. Albert lists it as button NJ-23.
Regards,
Pete [P.C. George]
