If I understand your question about the top "A" LM339 comparator that drives GRN, that comparator operates opposite the YLW and RED comparators. That is, when RED is high, the RED comparator turns on the RED LED. When YLW is high, the YLW comparator turns on the YLW LED. OTOH, GRN operates differently. When either YLW or RED are high (the "or" being implemented by the two diodes) then GRN is turned OFF (rather than ON). This is opposite so that's why the top comparator is different in polarity.
As for making it compatible with the PRR-style, it was just a casual comment. I don't think it is a big demand item amongst the DIY crowd - if for no other reason than I don't think we_honest sells a low-cost PRR signal head! The center LED is always ON. Each R,Y,G output drives 3 LEDs when active...two of the three are unique but the center LED is common. In any case I found this video of a common-anode 7-LED PRR signal with time delay implemented using eBay relay modules.
