Are you using a 37mm Lens? Here this FLD filter is with 37mm thread.
FLD is a filter for matching daylight film to fluorescent lighting (FL for Florescent Lighting, D for Daylight). On a film camera, you'd use it to correct for the different tone of light and give your shots a more natural looking colour.
37mm Thread FLD Filter Lens (Purple FL-D Filter Lens / Fluorescent Lighting Daylight Filter Lens)
Link to learn more:
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