Checked my pictures and I've got maybe six of them (dual reservoir master cylinders) marked on the drivers side back behind the rear most port. Colors vary from white to red. Some appear to be quick swipes of a marker while others appear to be paint daubs.
All but one is off the car and most are NOS with rebuilder stamps in the side of the master cylinder. Only one claimed on a car is one that is posted on Jack Brooks (appears to be a white mark) on a reported 7,000 mile 69 Shelby.
What I've current got on the subject