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While there's no definitive proof, there's some evidence pointing to it being at the Irving Big Stop in Baileyville, ME at the intersection of ME-9 and US-1.
Tesla's findus says Baring, ME, but that's tiny.
A report in a small subscription-only newspaper the Calais Advertiser had a headline about Tesla putting a Supercharger in Baileyville. Haven't read the report, so don't know.
Aulac, NB and Enfield, NS will both be at Irving Big Stops.
@shonline has stopped at the Baileyville and in the Kennebunk, ME and Aulac, NB Supercharger threads posted photos of Dig Safe markings at the Baileyville Big Stop that appear to be prepping for Supercharger-sized excavation. Supercharger - Aulac, NB Kennebunk ME Supercharger