I've successfully swapped my car's instruments and coding plug with a 1990 535im. The main trick is to make sure that the donor car is from the same market (US, ECE etc), has the same engine, transmission and fuel tank.

The early cars do have a coding plug, its just that it is very different to the later ones. The early one is a black thing that is embedded in the brown connector in the wiring harness for the cluster.