This cable is suitable for all 128K Spectrums, from the original right up to the +3. It connects via the phone-type RS232 socket.
The RS232 socket on the Spectrum has pins numbered 1 to 6, with pin 6 being the one closest to the little "lever" that you press to release the cable.
Z88 (9-pin) 128K Spectrum (phone-style) GND 7 -------------------- 1 GND TxD 2 -------------------- 2 TxD RxD 3 -------------------- 3 RxD RTS 4 -------------------- 4 DTR CTS 5 -------------------- 5 CTS