we just speculating here, but why it should be different design wise?
5 elements/ 4 groups speaks for a Planar type, which was very common in the 60's (Rolleiflex Planar, Nikkor F 50/3.5 macro and others) and the major companies kept the macro lenses unchanged well into the 70's...
Obviously the Pen F Makro lens was a "late" lens in the Pen F series. It wasn't in the lens brochure of 1968, perhaps it was released as late as 1970 (the demise of the Pen F series was 1972). The OM loupe lens 38/3.5, however, was released 1972 according to this info