I don't know. I do know that Joe Louis had a couple title fights scheduled for 20 rounds, but they both ended inside of 15. I also know that Max Baer won a 20 round decision over King Levinsky in 1932.
Baer actually fought another 20 rounder after that with King Levinsky, BUT Max is not in the answer. He actually fought the last 2 bouts that went the full 20 rounds. The bout I am talking about, went more then 15 rounds BUT less then 20.
Yes it was! Very Good. March 17, 1943. Lightweight Bobby Ruffin stopped Bobby McIntire in New Orleans, Louisiana in the 17th round. PS_ Actually "Duodenum" sent me the answer yesterday by way of a PM.