I have seen this mark on a fair number of 69 9" rearends and consider it one of the "normal" or typical ones. The problem IMHO is apply it is that its been found at the drivers end or the passenger end. Don't know why as I thought originally it was an orientation mark.
If I was suggesting I would suggest (given your specific application) placing a yellow mark /stripe all the way around the end of the rear end housing (basically where its welded to the flange) using a brush. This should be done with the rear end sitting up right on the drivers side flange (or moved to that orientation right after application) so that runs flow towards the drivers end of the housing