the index dot indicates that the indexing ring is 90 degrees into the 180 degree travel.
That gives you the correct position when you line the flat of the d bit up with the pull out blade to rotate the back edge for grinding the relief.
There is probably an index notch that corresponds which you can use to lock it whilst making the marker.
Have you dismantled the spindle? It possible the index ring has a place for the dot on it, but the hole in the housing to view it hasn't been drilled.
Dave