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That's the decimal point for when Num Lock is activated for number keypad mode.
 
WildCowboy is correct. The smaller characters on the 6, 7, 8, 9, 0, -, u, i, o, p, j, k, l, ;, m, ., and / take effect when num lock is on. Apple calls this the 'embedded numeric keypad'.

7, 8, and 9 happen to correspond to themselves, as does . (full stop/period,) but the other little 'lower right corner' symbols are to duplicate a numeric keypad of a full size keyboard. (The 'c' on 6 is for 'clear', not the letter c.) One of my major gripes about the Dashboard calculator is that its keys don't conform to the layout of a standard Apple keypad, including the 'embedded numeric keypad' on notebooks.
 
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