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The site plans show an accessible stall in the middle, separate from the prebuilt Supercharger units. The gap is likely for the extra lateral spacing needed.
Strangely the plans show the second prefab unit as a
three stall prebuilt unit with a standalone stall. Never seen that before. I couldn't quite tell in your pictures, are there 8 stalls installed right now (2x prefab slabs), with a gap in the middle?