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UPDATE
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Figured it out!
00001131 = EUR00001132 = USA
ext0 = Slot 0
ext1 = Slot 1
EUR -> USA:
- Decrypt your EUR save with SaveDataFiler
- Start a new save on the USA version of the game and decrypt this as-well
- Move
"SDcard:\filer\ExtData\XXXXXX\00001131\fl_ext0.fsd"
into
"SDcard:\filer\ExtData\XXXXXX\00001132\"
replacing the file that is already there. - Import save data from sd card "00001132" with SaveDataFiler
- Profit!