This happened in an existing apartment building. There was an MDP with main and feeders to several different basement areas. Each feeder was 200a and fed 5 or 6 metersocket/panels ganged together. From the metersocket feed to the panel was #3's. I made the contractor install the main in each panel due to the tap rule requirements. Yes it was a back fed device, and the contractor installed a bracket so the main could not just be pulled out. But he installed the main in the middle of the buss. It turns on and off horizontally, so I guess it sounds like this is ok.
