The origin can be found at the accelerometer. It can be found right under the letters se from Xsens. From the center of the screw closest to the X, go 15 mm to the right, 13 mm towards the connector and 7 mm up from the bottom of the bottom plate.
For OEMs the same origin applies; the reference point is then the left-upper hole in the PCB. The distance to the origin (top view) is 6 mm to the right, 1 mm down and 3 mm up from the bottom of the PCB.