Are the 915 heads hand stamp HP?
You need to keep in mind that Mopar had been using the low po 350, 355, non HP 440 motors in C bodies cars in 1966, not in the Dodge or Plymouth B body cars until mid year 1967
I've seen several original 1967 Dodge RT/T 440 motors, I don't remember if any of those had HP stamp on the upper pad or not back then.
Maybe the HP stamping started in 1968 along with the motor numbers hand stamping #s matching the body VIN #s