JimInOz
··Melbourne AustraliaMarvin Watch Co?
M.W. Co is probably the stamp of the British agent or importer of Leonidas in the mid 20th century I would guess.
I've seen a couple of other Leonidas with that stamp, all in the UK.
I think you need to take the dial of Kostas, if the ebauche maker isn't under the balance it is 99% stamped on the plate.
I'll keep looking and thinking.