600volts vs 1000volts relates to the safety category, and your old meter was probably made before the safety category rating for testmeters was introduced.
Have a read at the Fluke application note which TVtech has linked to in his thread: https://www.electro-tech-online.com/threads/multimeter-safety.143362/
Anyways i was just wondering how it would cost if i want to sell it as "vintage" since its from the 70's , but am planning to use it on the bench for the time being .