It looks a little like a variant of the old German Schmeisser machine pistol, or it might be some Eastern Bloc weapon captured from the VC (the AK-47 armed NVA would not have been around when Ellsberg was there). The Czechs, in particular, used to make some really nice weapons, they were issued to the Cuban Milicianos by Fidel. I'll have to ask my cousin about it, he may have had one just like it in his house. The VC were armed with all sorts of hand-me-downs, even gear they captured from the French.

Not being a real GI, but a civilian, yet familiar with firearms, Ellsberg may have had the freedom to choose not to pack an M-14 (too heavy) or an M-16 (too unreliable).