Install a genuine OKI ink ribbon in your ML520 or 521 Microline dot matrix printer for top quality monochrome black results. This ribbon is designed for an easy changeover for quick installation. The high quality nylon construction is designed for minimal wear, offering a lifespan of up to 4 million characters per ribbon. It uses premium-quality inks with a metered output for consistent quality, precisely controlled for even coverage for each and every character.