You need to consult the instructions and labeling of the panel.
"Electrical continuity" is not enough for a neutral bar. The current pathway must also be rated for full-load current, and that is mostly a thermal rating, i.e. can it carry full current without overheating? Lots of things that are good enough for a momentary 500A fault event are NOT good enough for a continuous 40A neutral load.
It is also common for main-breaker panels to be supplied with no grounds at all, you'd be expected to buy accessory ground bars if using it as a subpanel.
But they will always provide enough neutral lugs for the entire panel to be populated "maxed out" with 120V circuits. This looks like a 20-space panel (the breaker stabs to the right of the main breaker have been milled off so nothing can go there). I don't see any slots to accept tandem breakers, so this appears to be a 20-space/20-circuit panel. I count 16 screws on the right and 16 on the left. That's 32.
A slight variant of this panel (no main breaker; stabs not milled) was offered as a 24-space "main lug" panel*. That would require 24 neutral screws (1 neutral per lug, per Code) + 8 ground screws (3 per screw, per UL certification). That's just right for the main-lug variant. So I suspect both bars are intended for use as neutral... however, check the instructions and labeling: they are the last word on the subject.
UL approves the instructions and labeling as part of approving the panel.
* And if you ever want a generator interlock, that's food for thought. Find that version of this panel and swap bus assemblies (basically everything that isn't neutral bars, the whole shebang comes out with 2 screws). You don't even have to take the breakers off their wires, it's that easy. Then the top right 2 positions are populated and you can use it for the generator breaker w/ interlock.