CrossRoads:
Don't know about the traces - maybe its to help with tracking pirated boards.
1Ks provide some seperation so D0/D1 can be used with external devices.
Where are the unexplained via holes?
The via holes are located at the same place inside the yellow circle on the first picture (schematics screenshot).
Regarding 1K resistor separation from external shield, how does it work explicitly ? The main board or the external shield will get hold of the serial lines first and why???
I have made a mistake. M8RXD track is actually longer than M8TXD track. From my understanding, track impedance should be affected primarily by track width. Both M8RXD and M8TXD tracks have the same width of 12. Anything that I have missed out ?