"RP" stands for LMG in Russian. 3rd letter stands for designer's last name.
K = Kalashnikov
D = Degtyarev (sp?)
RPK IS an AK with a heavier barrel, receiver and a some cosmetic differences. Most parts are interchnagable.
RPD has no similarties with AK (or RPK). Completely different design, action, feeding not to mention that it was designed 3 years before the first AK-47. They have zero interchangable parts. SKS is actually much closer to an AK-47 than the RPD is.
There is also PKM, which was also designed by Kalashnikov but, like RPD, is not classified as a variant of AK-47.
RPD is not prohibited by name, and as long as it has a newly made receiver that cant easly accept full auto parts, it's good to go.
Whether or not belts will have to be limited to 5 rounds is a different matter.