#1 volume 1 is a really great issue (and obviously a handsome front cover,) but I would lean to Mirage TMNT 46 or 47 as being a true first appearance. Yes, as Ryan points out, #46 (April 1992,) is a promo type design spread for not only Space Usagi but the ships and other characters in the story arc and it does use four whole pages to show 'coming in June 1992.' TMNT issue 47 (May 1992,) has Space Usagi in an actual paneled story one month before SU #1 (June 1992.) I suppose it will be for collectors to determine which counts the most.