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Appears to be framing of 2x6 sitting on a mudsill with older 8" exposure cedar lap siding WITHOUT a WRB. Since there was also no WRB installed when they added the aluminum (or vinyl) siding with open joints at the corners, water ran down and wet it all. Appears they removed the corner boards or that one was cut short... need to replace the siding section and the floor joist rim board section, no jack required. You will still have a water problem there though.