The "oval" box simply contains a relay and some signaling electronics. Different NEMA adapters for that Tesla cable have different resistors in them. The box reads that resistance, and based upon it, tells the car to only pull up to 80% of that particular circuit. For instance, plugged into a NEMA 14-50, the box will tell the car to only pull 40A. If you plug in an adapter for a 14-30, the box will tell the car to only pull 24A. This is how all J1772 compliant (the Tesla uses this standard, but another plug to connect to the car) EVSE's work. The box also incorporates a ground fault detecting circuit to open the relay under ground fault. The box also checks for proper 120V from the supplied ground to each line, or 120V to one and no potential to the other in the case of a 120V outlet. Until the car signals the EVSE that it is ready and properly connected, no line voltage is downstream of that box. The key thing about these installations is to understand that they push the limits of NEMA outlets. If the wipes are not tight, or the lugs are not torqued properly on the receptacle, 40A continuous will cause issues (meltdown/fire). The car will charge faster the more power its permitted to pull, up to its own internal limit. The actual charger (switched mode power supply) is housed inside the car. Some EVSE's are cord and plug installations, some are hardwired. Interesting point, the 14-50 and 14-30 adapters, while having a neutral prong, actually have nothing connected to them. So if you are metering the neutral and think there is a problem, there isn't. As to why they didn't standardize on NEMA 6 series, I believe it was due to the ubiquity of RV outlets (14-50).