Well if the old one had the same issue and you're positive you have the 64 1/2 - 65 switch then I'll refer to my usual answer that electrical issues especially lights 99 percent of the time is a ground issue. So I'll try to keep this short. So to me that bulb looks like one of the few that operates with reverse polarity. It has two wires connected to it with one supplying power while the other feeds to ground. They then reverse power and ground direction as you change the switch direction. So this means they are likely using different ground sources and one of those is probly a poor ground which could be at any of the 4 blinker sockets or even headlight grounds. It may be poor but good enough to operate the blinker itself. You might be able to watch the blinkers and see if maybe one is not as bright as the others which would indicate a poor ground at that light. I would personally clean each turn signal socket ground and headlight ground. The front blinkers tie into the headlight grounds.
Just out of curiosity try turning the blinker on and then apply the brakes and turn the headlights on and see if the indicator flashes then
You can make up a test ground wire with a couple feet of wiring with aligator clips on each end (I always keep one of these on hand). Connect one end to ground wire of a socket and connect the other to clean bare metal on the car (preferably the frame) and then work your blinkers. Repeat for each socket.