Wow, my first post here at Retro. A couple of months ago I bought 4 shirts from Yahoo Auctions that had these artworks I've never seen before: 
Now I'm not sure if I'm right, but I think some of these shirts are from that store in Japan that sold only Sonic merchandise. I remember there was this person that had a blog which feature a visit to the store and it had some of these shirts. Sadly I can't find it, but there is a section about it on the Sonic Gear site. Looking at them, I've noticed that at least the artwork on the black shirt seems to be an alternative design to this one.
It makes you wonder what other artwork we haven't seen from Yuji Uekawa.

Now I'm not sure if I'm right, but I think some of these shirts are from that store in Japan that sold only Sonic merchandise. I remember there was this person that had a blog which feature a visit to the store and it had some of these shirts. Sadly I can't find it, but there is a section about it on the Sonic Gear site. Looking at them, I've noticed that at least the artwork on the black shirt seems to be an alternative design to this one.
It makes you wonder what other artwork we haven't seen from Yuji Uekawa.

