What are some clues that led Herman E Colloway to being bud's grandfather in Bud Not Buddy?

Herman E. Calloway was able to recognize Bud's keepsake flyer that was handed out by Bud's mother. He also noticed the specific writing on the flyer that matched his own handwriting, which hinted at their familial connection. Additionally, Herman recognized the photo of Bud's mother on the flyer, leading him to realize that Bud was his grandson.