Clearly those ribs could only be molded to produce a one-piece product if the two halves of the mold met in the center of the air spaces and the angled ribs. That would mean the mold being split vertically from the top of the butt to the bottom, so to speak.
If molded in one piece those mold lines are perhaps just artifacts of the composite(?) construction of the molds.
If the pad that contacts the shoulder of the shooter was somehow attached to the ribs after molding that would allow the markings we see on the commercial recoil pads like the Jostam to be pulled straight out of the mold as they would have to be to separate cleanly. That method would also allow the use of different colours, markings, pad configurations etc.
Another possibility would be a three piece mold and the line running along the base of the pad would be where the third part of the mold met the two halves the ribs and base were formed in.