you can use every flash on the M 6 which has the international ISO shoe. If you switch the flash to manual or simple auto operation (not TTL) the additional TTL-contacts are not operational. Since the M 6 classic does not have any electronic circuits for TTL inside, there cannot be any problem. As your flash is suitable for modern digital equipment, it has a low voltage between the contacts and this is even better to protect the mechanical synchro contact from erosion by sparks (but the M 6 is a good and expensive camera and I suggest that Leitz had them gold plated). Old Metz CT 45's and many other older flash units have over 200 Volts.