If you look at the first picture, (closeup of the neutral lug), there is what appears to be a combination flat / robertson head screw on the lug, that screw secures the neutral lug to the left hand bar. This is what allows the neutral to be electrically part of the neutral bar on the far left hand side of the panel.
If this screw(challenger guys need to chime in) also screws into the metal frame of the panel, it would be bonding that bar to the chassis, and electrically tyeing it into the grounding bar on the far right hand side of the panel. The question is, is this screw green in color or some other color. If its green, it is the bonding screw.
At the very bottom lefthand corner of your second picture…on the far left bar, where does that thick black wire that ties into the bussbar go? Does this go over to the right hand buss? This could be serving as your "bonding " wire.
Any chance of getting a bigger picture so we can see what is going on in the panel?
In thinking about this, if the main neutral cable were not electrically tied somehow into the right hand buss, there would be no power to any of the 120v branch circuits in this panel. (because in your second picture, all of the neutrals go to the right hand buss, but the main neutral that feeds this panel attaches to the left hand bus. Somewhere, the left hand neutral bar is electrically tied into the right hand neutral bar…either by the screw at the lug, or a wire that connects the two.