The sole exception after 1942, as far as I'm aware, being the booklet "Defending and Lagally Establishing the Good News" in 1950 which bears the Watchtower imprint and names Covington as author.
Some have suggested that Knorr made the books anonymous because he didn't write them and didn't want to give others prominence. It's also been suggested that Knorr disliked Convington in particular for the prominence that resulted from his legal work for JWs.