Here´s a photo captured aboard Gneisenau while in transit through the Nord-Ostsee Kanal in August 1939. Take a closer look into her mainmast - over the funnel outlet at level with the converging point of her tripod legs. Are these triangular signals the same ones as you noticed in the aft mast (main) aboard the SMS ships...? If so then yes, its the rudder position indicators.
Note that these signals had an onboard use rather than as signal between ships. They would serve as a quick reading of rudder position to any man aboard- within sight of the mast of course. Both triangles would usually be seen either at level with each other; rudder midship, or in a diagonal sort of appearance (high and low) during turns. For the record, I`m compelled to believe down is synonymous with inwards rudder position - if port side triangle is lowered then port rudder is performing a starboard turn.
And just to have mentioned it before someone decides to ask, the `box with the cross´ lowered from the yardarm at port side of the foretop artillery platform probably serves as outboard signal that the "ship`s maneuverability is impaired"