Do you have information supporting that M.V.M. refers to a ship? I think the "G.N. C.oy" refers to the "gunner company" (company as in a military unit). Multiple sources confirm that "Coy" is a military abbreviation (not like "Co." to indicate a company in the business-sense), and in military language, G.N. is for gunner.

no; I have previously been trying the Massachusetts Volunteer MIlitia with little success and here in Australia this line has been ridiculed as too far removed, I do known of a second piece here. Ps I thought maybe garrison coy.

M V M is 1895 in Roman right?

M = 1000, V = 5, so if anything, it would be MVM = M + VM = 1000 + (1000 - 5) = 1995. But it's not, since that would have been represented by 1995 = 1000 + 900 + 90 + 5 = M + CM + XC + V = MCMXCV. Interesting idea though; roman numerals didn't pass my mind yet..