I am pretty sure it'll be on the water table and/or the barrel flats. The closest thing my lists show for Gmos would be the initials G.S.M. within an oval ring, indicating made by Gregorio San Martin. Unless yours takes that exact style, it's likely an incorrect identification, so keep looking. You might also look for the date code, which will appear as a capital letter followed by a single number. 1970 is O2, 1971 is P2, 1972 is Q2, etc.