Item C Stock block refers to a block of stock with a unique cost basis. A separate IRS forms 7203 would be prepared for each stock block.
The IRS Instructions for form 7203, page 2, states:
Item C
When a shareholder has a different basis in different blocks of stock, pass-through items are generally allocated pro rata to all shares, regardless of their different bases. If there is a partial stock sale or partial redemption, you may file more than one Form 7203 and provide a description of what period the form covers. See Regulations section 1.1367-1(b)(2) and (c)(3) for details.
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