On my old metal SAE bolt diameter x thread size chart it says:
7/16 x 14 coarse thread SAE
7/16 × 20 fine thread SAE
If you have a thread pitch gauge, you may be able to fit into the hole to check the threads with a flashlight to see what your viewing. As far as a number 24 thread, nothing 1/2" but I am pretty sure there was a 24 thread on the 9/16" diameter, but it would not be that size.
Ah, the crooked seller 72Dart6pack. Sold me an OER radiator for my '72 Dart, that didn't come close to fitment and also had holes punched into the cores from trying to fit a shroud. Wouldn't respond to me, I lost $500.