You're correct about the TUL-1 using the AK milled receiver, but that's a stamped receiver on that particular RPK. And the RPK receivers are made stronger than those of the lighter AKM; they're built at the old Molot PPSh-41 *Shpagin* SMG factory in the Kirov district. Additionally, the Vietnamese didn't use the RPK stock on their TUL-1 guns, but rather the regular short but of the parent AK instead, probably a better fit for the average Vietnamese soldier.
-archy-/-