The ones I know of are minor frame strengthening, hammer rebound spring, slightly heavier (green) recoil spring for plus P plus loads, 8 round mag capacity, and adaptability to DAO operation.
I have the 8 round followers but prefer 7 rounds under less stress. Had mine factory converted to DAO but went back to DA/SA.
Went to SIG armorer's school some years back; the differences between a 1990 'American' style 220 and the current model are nice but not critical.
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