The LC 580 (a.k.a. Performa 580 and 588) is essentially an LC 575 with a new logic board that supports an IDE hard drive (instead of the more expensive SCSI hard drive used in earlier 500 series Macs) and uses a less expensive monitor.
The LC 575 (a.k.a. Performa 575, 577, and 578) is essentially a Quadra 605 motherboard in an LC 520/550 case.
The first cable-ready Macintosh! No, not ready for a cable modem – ready for cable TV.
The LC 550 replaced the LC 520, increasing CPU speed from 25 MHz to 33 MHz. It was released at the same time as the 68LC040-based LC 575.
Take an LC III and graft on a 14″ Trinitron monitor along with stereo speakers. That’s what Apple did to create the 520.