I think block refers to a Matroska block here, i.e. setting this should not only set the stereo flag to 13/14 but it should influence the actual muxing. I think this might be needed to keep it synchronized in case of seeking or error (so player always knows what's left and what's right view). But I'm note sure if that's really how it works nor whether mkvmerge actually does it correctly (testing mkvmerge should not be difficult, though).
I will think about it some more...
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