Replacing 60-year old light switch - which wires go where?
Based on the photograph, I'd say the black wires are feeding voltage from the panel (hot) and carrying the voltage onto another switch or outlet. The red wire is presumably going to the light (load).

You need to connect the black wires to one screw of the new switch and the red to the other. Leave the green (ground) screw unconnected.

It's generally not good to put two wires under one screw as was previously done. Better to add a pig-tail: a third short wire to the switch. Connect the two blacks and the other end of the pigtail together with a wirenut.