An update

Yesterday I swapped the #1 plug wire from my '73 into my '70. No flash on the timing light. I then swapped in one of the MSD coils from my '73 into my '70. I then started getting an intermittent flash on the light. Not steady. I reset the inductance pickup to more firmly cover the plug wire (that I had swapped back to the original) and got enough of an intermittent flash to set the timing. I then put the original Bosch coil back in the '70 and no flash. My conclusion at this point is a bad timing light. I will try to borrow a known good one one locally to see if it works on my '70.