Cari has it right. M is for Monza, Italy (1934-1968).
The nearest thing that I can think of that may have given newguy a reason to associate M with Spain, is the M.R. decal that Singer had to put below the Singer Badge on machines that they exported to some Spanish-speaking countries. The M.R. stands for Marca Registrada and means Registered Trademark. I don’t know for sure that Spain was one of the countries, but it could have been.
CD in Oklahoma