What do you think is the meaning of the writing on the rocks in Bud's suitcase?

In Chapter 8, Bud takes the five rocks out of the tobacco bag that he keeps in his suitcase to make sure they are all still there. Bud mentions that someone had taken a pen and wrote a secret code on each of them, and he does not understand what the writing means. The rocks say, "kentland ill. 5.10.11, loogootee in. 5.16.11, sturgis m. 8.30.12, gary in. 6.13.12, and flint m. 8.11.11" (Curtis 79). ...

In Chapter 8, Bud takes the five rocks out of the tobacco bag that he keeps in his suitcase to make sure they are all still there. Bud mentions that someone had taken a pen and wrote a secret code on each of them, and he does not understand what the writing means. The rocks say, "kentland ill. 5.10.11, loogootee in. 5.16.11, sturgis m. 8.30.12, gary in. 6.13.12, and flint m. 8.11.11" (Curtis 79). One can surmise that the names written on the rocks are locations in the United States, and the numbers are specific dates. Whether or not Bud's mother visited those places to watch Herman E. Calloway and his band play is unclear. The locations and dates could also be places where Herman E. Calloway performed. He could have possibly given the rocks to Bud's mother as a keepsake.

Later on in Chapter 18, Herman asks Bud to pick up a rock, and when Herman opens his glove compartment, Bud sees a bunch of rocks with similar names and dates written on them. Later on, Miss Thomas tells Bud that Herman picks up rocks wherever he performs for his daughter, who is actually Bud's mother. When Bud's mother was four or five, she asked her dad, Herman Calloway, to bring her back a rock from Chicago. Ever since then, Herman picks up a rock for his daughter wherever he travels to perform.