There’s a considerable back-story to this unused maximum card, and we are absolutely unable to explain it.  We consulted Hong Kong friends who said, in effect, nobody in Hong Kong now really understands this calendrical cycle well.  It’s apparently an old tradition in Mainland China.  Please don’t hold it against us that we can’t go much further than this, but the (Chinese-only) text on the reverse caption sounds something like “Mao Zi”.  It’s China Post’s card MC(E)-9 and postmarked 2008 but only went on sale in the Shenzhen post office in 2018.  Confused?  Not just you!  Grade: 1