Around the same period we had the F1. Same deal, mostly used for pay parades, in those heady days when you still got paid in cash. If you drew the short straw you stood around with a load magazine in your pocket (not on the weapon itself, that wouldn't be safe) whilst people marched up saluted and declared 'pay correct', even if it wasn't, after a cursory look in the brown paper envelope bulging with $$$.
As a weapon of war, it probably would have done most damage if hurled at an attacker. Legend held that a wet blanket was sufficient armour against its missiles at anything more than 50 metres. Fine chance if you could actually hit anything at that range. Still, you could fit a bayonet to it. We regarded this as being supremely optimistic, fitting a six inch knife on a submachine gun shorter than one's own arm. Upon mature reflection, it was a coded message as to the weapons actual effectiveness.
Yes the Yanks, principally the CIA, used the Carl Gustav SMG in Vietnam. In American circles I believe it's generally known as the 'Swedish K'. I think a number of armies used it, the Irish had them in the Congo, IIRC.