1976 Dodge Truck No Spark

I've got an old truck that I like and use around my farm, but today it stopped running. I've replaced coil, distributer cap, rotor, and ballast resistor, but still no spark. And what the heck is that brass
colored button on the inside of the driver's side fender?
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Chuck
I had a 72 for 20 some years. My guess is that the pickup coil in the distributor is open. Or intermittent with possibility a bad cable connecting it to the ignition module. Maybe a picture of the "brass button" is in order. Bob AZ
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