The Russians put the RPK in service using the heavier stamped receiver, and if that is the look you want, then the Yugo uses that idea. But no, not all RPK's from all countries using Kalashnikov designs, went with the heavier stamped design. The Bulgarians and Chinese both built their own variants of the RPK on milled receivers. If there were no "real" RPK variants in the world with a milled receiver, then I would say it would not be correct, but when you see it on an Arsenal Inc RPK, which they base on Bulgarian weapons, then it is "correct" for that particular version.
Just like the milled receiver is "correct" for their SA M-7 rifles, which is also based on a modern Bulgarian design that uses this type of receiver.
The Yugo stamped receiver is the proper and "correct" method for the Yugo RPK that it is based on. And of course, closer to the Russian method of building RPK's.
So the answer is yes, the Arsenal Inc RPK is based on a "real" RPK variant. As one would have expected the Arsenal Inc Company to do, they based it on a "real" Bulgarian design.
Thats one of the neat things about the AK-47, AKM, RPK, AK-74, etc, they may have originated in Russia, but the designs did not stand idle, instead there were variations of the originals througout the Communist Block and China.
A good example of this is the Russian SKS. While most countries pretty much stayed with the original pattern, the Yugoslavians decided to add a grenade launching capability and night sights to one of their variations. They even made theirs to not have a chrome lined bore and chamber.
Now, do I think that it's not a "real" or "proper" SKS because of the deviations?......No, I actually find it more interesting than the original Russian SKS design......and everyone knows that it's still a real variation of the SKS. I have one, and love it :)
By the way, do the Yugo RPK's have a chrome lined barrel? I never bothered to check to see if they did. I just took it for granted, and guessed it would be chrome lined. Since their SKS' do not, one never knows :)
I think you won't regret either variation, but for the money, I would go with the Arsenal Inc gun, and then buy a standard SA M7 rifle with the money I saved (since the Arsenal Inc gun will cost you hundreds less) :)