I was sent this device by a mechanic who found it behind the instrument panel of a car. When removed another 40,000km appeared on the clock. The unit appears to "massage" the data between the ECU and the display.

The design of the circuitry is very functional. Basically two CAN (Controller Area Network) chips, a processor and some power supply components. The design is actually very neat.
The real work will be done by the processor, and I'm not sure whether the unit passes all other data through untouched except the odometer data or if it buffers everything.

From the type and spec of the processor I'd guess that this has been developed on an Arduino style platform, but I could be wrong. It just has that modular look to it. The Dupont style sockets and the way one has been hacked into a very robust plug is also a clue that this was a geeks personal project that escaped or evolved.

The keywords to find these online are "can filter 18 in 1". Here's an eBay search link:-
https://www.ebay.com/sch/i.html?_from...
Keep in mind that their function is to reduce the apparent mileage of a vehicle, and this may be a criminal offence in some countries.

If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:-
http://www.bigclive.com/coffee.htm
This also keeps the channel independent of YouTube's advertising algorithms allowing it to be a bit more dangerous and naughty.