What we've been told by Ford plant workers is that each section of tubing (when cut) provided 3 Mustang drivelines.
As mentioned the base number identified the type (length, dia and thickness) of the tub - 19078
I believe that the last two - 4s for example identifies the date. It does not match the typical date codes from Ford since its not one of their codes and that two exactly the same cars produced at different times of the production year have shown up with the same base code - but the only difference has been the last two digits.
At this point I know of no one that has found any documents or worker (of the steel tubing plant) that breaks it down all nice and neat at this point