Mike, very few voice actors at that time got credit. Mel Blanc did because he went to Leon Schlesinger asking for a raise. Schlesinger said "Why would you want a raise? You'll just be in a higher tax bracket. I'll give you credit instead." It was definitely a contract agreement because other voice actors working with Blanc never got credit, including June Foray, Arthur Q. Bryant, and Stan Freberg. I grew up thinking Blanc did ALL of the voices, but that wasn't the case. So everyone from Donald Duck (Clarence Nash) down to Michigan J. Frog (whose tenor voice's name is lost to history) got no credit until much later in life.