There were discussions about this here before;
One idea was that when cars were built on the line and the installer did not have the exact numbered radiator that the build sheet called for, they substituted a radiator that was similar and would fit. They would then solder on a tag with the correct number and install it.
The other theory is that the tags were soldered on in the Mopar parts aftermarket by Chrysler rather then continuing to manufacture all the different radiator castings, since there was a lot of overlap on the applications of how small block and big block radiators could be used.
Not sure who would know the accurate answer to this?