The design of your bike is over 50 years old. Bikes have come a ways forward in design capabilities. You are missing out. Period. If you like the design of your bike so much why this thread? Rock your suicide levers and call it good. 21st Century interrupter levers have a much more positive engagement since they act directly on the cable, and don't have to transfer torque through a long (flexy) lever, into the primary levers, and then into the cable. Either one works. The modern version work far better.