I can't believe that the coding plugs are no longer available,
however there is very little in them to justify the price. I'm looking at one out of a W124 automatic and there are only 4 precision resistors on a small circuit board. If someone had the coding plug that you need but was unwilling to part with it, if they would be so kind as to tell you the values and positions, you could modify one very economically. The resistors are 1/4 watt 1% and should cost about 50 cents each from an electronics supplier.