If you look at the manual for a 568, it clearly shows an RS232-style D connector in the expansion socket.

Wonder what Meridian had planned for that socket?

500.2, 518, 568.1, DSP5000 (24/96), Manticore Mantra T/T, Rega RB300 Arm, Sumiko Blue Point Special MC Cartridge, Musical Fidelity X-LP Phono Stage, AKAI GX-95 MK-II Reference Master Cassette Deck & Marantz ST-17 Tuner