The HP Cassette Sub-Guide Rear C3510 is a replacement part for the rear guide section of the printer cassette. Its exact part number is JC90-01601A, which identifies this specific cassette sub-guide.
The parts list places it within the C3510 cassette assembly. It is intended for that assembly as a component, rather than a complete cassette. This part is useful when replacing the rear sub-guide while retaining the rest of the cassette.