Here's a question for you all.
The sons of the primarchs have 2 hearts (and all those other implants associated with becoming space marines). But -- do the primarchs themselves has those very same implants? Given how they matured to adulthood on wildly distant planets away from Terra, were they retro-fitted with such implants? Or have they never received them? In the background, the implants must all be implanted during the years of maturation - clearly missed by the primarchs. But yet, surely they must have the black carapace, otherwise their power armour would not function correctly?
Ah well! Love to hear your thoughts on that conundrum!