the SV that everybody is looking for is 99% certain French - and it is unlikely to be ValÃ©rysthal who used either ValÃ©rysthal in full, or PV for Portieux-ValÃ©rysthal. The mystery remains.
The Sv. in the Slovakian factory name is the Slavonic abbreviation for Sveti - Saint, so that does not bring us any closer. Thanks Max for looking it up.