I also do not understand where the difficulty is to draw a line to where the b-color goes, as there are absolutely unique piece: So this!
By the way, my observation is, that there is the b color only with Berlin-postmark, which suggests that, the sheets of this print edition in color b, only was delivered to the OPD Berlin.
Other stamp locations are not known to me! All this supports my finding that there is a partial edition, for a different color mixing was used!