These "mini transformers" are dry-type autotransformers (i.e. single winding) rated for 480V one side, and 504V on the other. I've seen them used at one other location, at El Paso. I can't figure out why these are used at some locations, and not others. Schematically, they would reside to the high side of the main rectifiers. Maybe an enhancement to ultimately get more voltage on the DC side, and hence more power to the cars? Or maybe they are a standard part of all newer installations, and all the other superchargers I've been to are older. Anyone else know? The serving utilities should all be providing 277/480V three phase to within plus or minus 5%, in accordance with ANSI.
Maybe these locations are known to be to the lower end of that band, as might be the case if they are toward the end of the distribution feeders, and Tesla wants a more consistent, higher voltage output on the line side of the rectifiers. You have to be careful then that it doesn't go too high during "off-peak" times.