Coast Guard Captain of Port ID card

I have a "Captain of Port" ID card for Biloxi, Mississippi, that belonged to my great-grandfather. His name was William Gray Slay, Sr. It was issued by the U.S. Coast Guard. It's signed by W.C. Rheingans, C.Q.M. It gives occupation as mail carrier. I'd like to find out when this was issued--looks like around 1920s maybe. Would there be other records available relating to him?