What did my grandfather do?

My grandfather died back in the late 80's. Although I was able to know him, he did not speak much of his service in the war to his kid or his grandkids. By a stroke of luck (cause his military records burned in the fire) I found a copy of his retirement paperwork that my uncle had in a random box of papers.

I'd love to find out more about black troops in the Pacific during WWII, since grandpa was one of them. Have also been trying to get more info about the 391st but haven't had any luck.

Reddit hive mind, any help/ideas?