On the M1967 modernized version of the M1956 web equipment, the shoulder hooks on the "H" harness is to attach a butt pack. That requires an additional adapter, however.
The M1967 nylon sleeping bag carrier (spaghetti straps), butt pack, and suspenders adapter for the butt pack were intended for all troops not issued the tropical rucksack ("X" frame version).
The M1967 nylon web gear is an indermediate step between the M1956 canvas web gear and the ALCE system. Components include the nylon pistol belt with stamped quick release; 20 round M-16 magazine pouches; nylon "H" harness with hooks for using a butt pack as a 'pack'; nylon cover for the trifold entrenching tool; tropical rucksack ("X" fame version); etc.
The M1967 webbing never fully replaced the M1956 canvas system. Most troops were issued full or partial M1956 gear right up to the fielding of the ALCE system.