Yes, I understand. But the original 915 heads didn't have the extended pad under the exhaust for the emission holes. Apparently Chrysler decided to recast the 915, for whatever reason, and emission-related heads with the extended pads were already being made for other applications with different numbers in the newer, then available molds.
Later castings also relocated the casting number to underneath the intake ports, plus, it seems that the first number of the casting is also missing on the later 915 heads that I've seen.