I have a 70 parts book also...I remember having a 70 440-4 Challenger R/T ragtop with the skid plate. All 70-71 HP 440's (no diff between 6bbl and 4 bbls)have the 6 qt pan (part # 2951564) that they share with the hemis. Same pan for B and E bodies. So if the reason is the 6 qt pan, then they all should have skid plates.
It would not surprise me if a 440 car was built without a skid pan K member, if the car came down the line, and there were no skid plate K's available, they'd use a standard K and move it down the line.