New reply from Florent Gorges :
Well, MU-21 is a private joke with my wife ^^
In Japanese, MU-21 can be readed "Munii". And Munii is the favorite rabbit puppet of my wife (when she was a child)
that's all ^^
one detail, the book and illustration copyright is not Pix'n Love anymore. It's "Omake Books Publishing" 