------------------------ UPDATE ----------------------------------------
The meters I got (like those in the OP) are exactly the same size - the second unit (the DC meter) snapped right into the box just like the first one. Both fit nicely, but I did have to remove the 'bottom' horizontal rail of the box. But they are mounted very close to the floor, so the install works fine even with the bottom rail removed.
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And now that I've been down there taking this picture, I think we'll spend some time vacuuming today!