Another very mysterious brand.
Before coming across the derailleur in this collection I had only ever seen an FM derailleur once - fitted on an Esmaltina bicycle that someone had brought to Britain from Brazil. On this basis, I assumed that both Esmaltina and FM were Brazilian brands. I now have been told that Esmaltina is based in Portugal - vaguely pointing to Portugal being the home of FM as well.
It is probably irrelevant but Maial is a town in Spain, which is fairly close to the Portuguese border.
The previous owner of the FM FMaial derailleur in this collection was French, and thought that FM was a French company.
I have never been able to find any solid information - so your guess is as good as mine - and my guess is that FM is Portuguese - but as Manuel would say, the reality is that ‘I know nurtheeeeng’.