Ahhh the infamous ESB ammo snap.
Indeed it is a snap, there could be at least three pouches with the snaps. I can verify at least two. One snap on the first pouch on the left side shown here:
There is also a confirmed snap on middle of the three pouches on the right side:
Now believe it or not, the Marmit ammo belt suggests there is a third snap on the fourth pouch/last pouch on the right side(so snap, two pouches with no snap, snap). Seeing as they got the other two snaps correct, there is a strong chance that the third snap is correct, though there is no image to support the third snap at this time.
The only version of the costume I have seen these snaps on is on the 3 pre-pro version, also known as "soon to be ESB Fett."
Though the pouches are arranged differently, you can see a snap in the front shots of the belt and a snap in rear shots of the costume (Chronicles).
As far as the snaps being responsible for closing the pouches...I really don't know on those particular pouches. After looking at the ROTJ ammo belt pics from both MOM and AOSW and talking with Brak's, it does appear that those pouches are glued down permanently. There is a yellowish residue around the closures, and it is probably the only way those suckers stay down so nicely.
