No real time clock.
No switched/backup ignition control wire (positive feed only).
Differential audio outputs instead of single-ended. This has the knock-on effect that the audio wiring is slightly more complicated than normal (it needs earthing cable inserts to ground the outputs to the same output ground as the amplifier).
No rotary control knob.

I am something of a fan of the Mk1; I have number 00015, and wouldn't part with it.

One of the few remaining Mk1 owners... #00015