The first "bag is a 1910" haversack as stated. It was the main issued pack for the US infantry during WWI just as the m1928 haversack was the main issued pack for WWII. They differ mainly by having one less / more rear support hook and the mess kit pouch on the WWI is a button fixture where the WWII pack has a strap. It is a pack system opposed to a bag, and it can extend to accommodate more equipment.
The musette bag is a nickname for the 1910 or 23, 36?... (cant recall the exact term) bag. It can be used as a shoulder bag with a strap, or like a back pack by using m1936 suspenders and a pistol belt. They can not be worn alone like a backpack, and if they do you might have a repro. The musette bags were given to mainly NCO / officers and special force troops (airborne etc.) during WWII. If you study original pictures you can see many cases where regular infantry would ditch their haversacks and use their gasmask, gp, and sometimes a musette bag for their equipment instead.