The Brother LC223 LC225 LC227 refillable cartridges have been shipping with small black plastic pieces. In the vast majority of cases these will not be needed, but are included with the package to cover all of the bases. There are two different kinds of original Brother LC 223/225/227, and a small percentage of cartridges use a slightly different shape cartridge, and the small black plastic pieces fit onto the refillable cartridges to contact with the printer sensor.
Below are fitting instructions. They show how and where to remove and replace the plastic sensor piece.
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