I've posted history behind the L and MB series guns which were made when the prices of the K's did not support the cosmetic touches that the newer refurbished K's are getting, i.e. welds smoothed out, blued parts etc.
Not sure why a welded tube vs a Pearl type construction make a difference. In my layman's logic, the receiver of the K is subject to almost no stress at all which is why they seem to last forever. Same for most blowback (impulse) operated SMG's (although the M3 had some issues with the center weld failing with all the bolt pounding). The tube just contains the spring, bolt and ejector.