You are correct.
The closure system is the key to IDing the US bags.
If it has a lift-the-dot as these do , they are SMG spare mag bags. The WW2 ones were for Thompson 30 rd mags and too short for the M3 . Post war , they made them longer to take the M3 / M3A1 Greasegun mags , but of course they would take the Thompson's too. They were spec'd to take 6 mags , but several WW2 mfgs. made them a bit large and you can squeeze in 8 .
If it has a strap and buckle , it's a GP bag , and was used as a catch all . Mags , clips , cardboard boxed carbine ammo , enblocs , ammo cans ( yes , a can of 250 rds belted Browning ammo fits ) , hand granades , rifle grenades , whatever needed carring . This bag had an internal vertical divider to separate the load ( say half rifle frag. grenades and half AT rifle grenades or flares ) , but it was floppy enough to be shoved to the sides to hold the can of belted ammo.
Chris