The product is a set of 6 Porelon nylon ink ribbons specifically designed for Okidata printers ML 320, 321, 390, 390+, 391+, 590, and 591. These black ribbons are compatible with Okidata printers and are sold as a single unit. Porelon is a trusted brand known for producing high-quality printing supplies, making this product a reliable choice for those in need of ink ribbons for their Okidata printers.