I did read all documents.
In the case of the DB820c, where the connector is in the middle of the board.
That page does not help to determine pin 1 on the DB820c.
This should be starring in your face since connecting extensions backward could be fatal.
I have Head Strap Magnifier with Lights and 5x magnification and 10x magnification of pdf to see the blurred numbers by the cross pattern background :-).