The locality number will depend on your specifics. For Winfield, Illinois, it appears to be Locality 1.
To find your Locality Group Number, see page 18 of the Form 1040 Schedule A instructions which lists localities by state.
Correspond your locality of 1, 2, or 3 to match what is listed of A, B, or C, respectively. For Illinois, it appears that A corresponds to 1 and B corresponds to 2.
To confirm the amount, use the IRS Sales Tax Assistant to enter your particular details. The IRS Sales Tax Assistant page link provides an overview.
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