It looks like a 26s 3p module, meaning that each group has three cells in parallel. That means that at each end of the module there is a set of cell terminal tabs for three cells (the left end of Group 1 in your annotated photo), and at each connection point in-between there is a set of cell terminal tabs for six cells (three from the positive end of one group, and three from the negative end of the next group); the cell tabs are connected together with metal channels and you have noted the sets with boxes. The channel which bridges the negative end of one set to the positive end of the next set (so the boundary between cell groups) looks different because it has a texture. So where the red meter probe is located is the negative end of group 1 and the positive end of group 2; the meter is measuring Group 2, and what you labelled as "Group 2" is the third group.Oh, thanks! I am not sure exactly how the cells are grouped, but it seems like the best hint is those little holes stamped in the middle of the rows, are exactly 26 holes, and I know it's a 26 series battery. Should I be charging them like I illustrate with the colored squares? The way I'm holding the multimeter probes in the picture was giving reverse polarity to what was marked on the battery, but by placing the probes in the matching color areas it was giving 3.3v at polarity that coincides with the battery markings.
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Each of those sets of cell tabs are connected by a wire to the BMS - you can see the BMS wires connected at each of them, on the part with the hole or the adjacent part (with the texture).
The same modules (made by A123 Systems and used in Smith trucks) appeared in an earlier thread, again with a discussion of the layout:
Question about balancing 26S/3P, 60 Ah A123 Brand 5KW modules
Other A123 modules appear to be similarly constructed, such as these in a for sale ad:
For Sale: A123 Battery Modules 10s3p and 13s3p with MBB & BCM (Michigan)