Complex sentence: The design was based very closely on the existing American 240 Volt plug and socket outlet design and it seems that the same American design is the reason that Argentina, Uruguay and China are now also using (essentially) the same design.
Split into simpler sentences: The design was based very closely on an existing American 240 Volt plug and socket outlet. It seems that this earlier American socket outlet is the reason that Argentina, Uruguay and China are now also using (essentially) the same design.