The lower Pack carrier of the M1910 and M1928 haversacks were to carry the blanketroll/shelter half/pegs. Experience in WWI showed that wearing and moving about with the fully loaded haversack/pack carrier was very cumbersome and uncomfortable. probably, by WWII, you see just the upper portion haversack being used. I am very surprised that the US Army did not adopt packs
based on the double and single bag packs that were prominent during the American Civial War. With those packs, a variety of items could be carried inside, there was a flap opening at top...not the hook and strap affair of the M1910......and a blanket could easily be carried strapped to the top. I guess by time WWI came about, soldiers were not expected to have full marking gear, but just enough to last a few to several days in the field. The Marine Corps M1941 pack was very good......which I think the army did take note of when they designed a new pack in 1944/45.