What dose Leo learn about the porcupine necktie at the end of chapter 20?

At the end of Chapter 20, Leo finds out that it was Stargirl who had left him a porcupine necktie two years ago on his birthday.
As Leo and Stargirl start discussing a gift she was thinking of leaving for a patient at the hospital, it dawned on him, most likely, that she had been his mystery gifter years prior. And it makes sense. When Leo first moved to Arizona, his mom had told the newspaper that her son collected porcupine neckties. This was true, except that Leo could never find a porcupine necktie to add to the "collection" started by his uncle's gift (which was the only porcupine necktie Leo had ever seen period). When Leo started going on his missions with Stargirl, he realized his gift of the necktie is most likely something Stargirl would mysteriously leave on one of her "missions." Leo wondered where she had found the porcupine necktie and she finally explains that she had asked her mother to sew it on for her.
By the end of Chapter 20, Leo tries to convince Stargirl that it's nice to get credit for gifts. More importantly, he tries to explain that people expect to know who leaves them a gift which she didn't see the importance of before that conversation. The fact that Stargirl leaves gifts anonymously is, eventually, what gives him a clue about who the mystery gifter of his tie was, two years back.