LED Indicators/blinkers, side/running and tail/brake lights

Are you sure the 2 flasher terminals are reversed? I found that the schematic in my 1965 Dart Shop Manual is misleading, depicting like you are looking at the wire side of the connector with round flasher in background, but actually it is for looking into the end of the connector (unplugged from the flasher). You could measure the wires with flasher unplugged and key off. The "X" terminal (black wire in mine) should go to ACC feed and "L" terminal to turn signal switch (red wire in mine). In an older thermal flasher, perhaps polarity doesn't matter since it just heats a thermal switch to open and close. Might not even matter in some "electronic types" (if no semiconductors and just an internal relay and resistors).