The coding plug is basically a number of resistors of different values that are connected between the pins in different orientations to designate the version.
I emailled you the diagnostic for a 1990 300e and just realized that you said 300D. I'm sure they are different. I will try to look up the right one and resend.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician