Hey, another AM3 Sonic. One thing that immediate jumps out to me is Sonic's render on the first page. I wonder if Uekawa used that one drawing as reference for his concept art?
Seems like it's a pose that they reference a lot. And here's a pencil case that I also scanned. Link to scans. Seems that artwork for Sonic, Tails, and Knuckles come from that SegaSonic style guide someone here was looking for.