Minitrix 12865 from the mid-1990's contains an NEM 651 socket has been solved!
As a reminder, the Minitrix KLM-liveried Koploper's were produced from 1994-1999, which was the very early days of DCC and as far as I know, a long time before any manufacturer had ever made plans for "DCC ready" trains (which is what the NEM 651 socket would allow)!
Some more explanation of the above terminology may be useful for some readers: "NEM" is an abbreviation for "Normen Europäischer Modellbahnen" or "European Standards for Model Railways" as we might say in English. The NEM Standards are set by an organization called MOROP, which is sort of a European version of the NMRA (National Model Railroad Association). When terms like "NEM 651" are used, we are referring to the standards set by the MOROP organization for a 6 pin 'plug and play' DCC decoder interface that was nearly universally used by all European N Scale manufacturers until recently (here is the PDF standard; in German of course!).
Yes, I said 'was' the universal standard...see this excellent overview of all the new 'standards' at Pierre's excellent Digital Train Blog for the current chaos in decoders!
Okay, back to the story. A couple of weeks ago, Baz - a Singapore based reader of this blog - sent me an email asking questions about my Koploper's. Baz wrote about his enthusiasm when he acquired a KLM Koploper, but was a little disappointed to discover that - unlike the version I had in my review - his Koploper did not contain any NEM 651 decoder interface!
Here is where I have to give Baz a huge amount of credit for not only discovering why his Koploper was different than mine, but also figuring out how to make it that way!
What Baz discovered, and shown in his photo below, is that his Minitrix 12865 is equipred just like they are shown in the original product manual and explosion documents (visible here).
link...unfortunately, I've not found that the Märklin or Trix 'stores' are set up for use by folks in the US, and rather we would need to go through a dealer or (yikes!) Walthers?)
And, just to prove that point, below is a photo from Baz of the 2004 catalog Koploper 12749...with the green circuit boards and decoder interfaces!