The first 50 White circles published in 1942 had text only covers. Illustrated covers were introduced with the spring 1943 books - numbers 51 through 72 plus 101 to 110. None of these books had signed art work. Starting with the fall 1943 books, number 74 on, Margaret Paull became responsible for the covers.
We know that Franklin Carmichael worked on three hard covers for Collins and that he was responsible for the hiring of Paull. An article in the trade magazine Quill & Quire (May, 1985, p. 20) says that after a short time at an advertising firm Paull joined Collins in 1943 when Carmichael, who was one of her former OCA teachers, “called her one day and suggested that she look into a position at [Collins] designing covers for and producing paperbacks”.
So it is possible that he was responsible for the paperbacks until someone could be found to take over on a permanent basis. The best evidence is the books themselves which show a sophisticated design hand. The covers are among the best Collins produced and significantly different from their American counterparts. Here are a few of them.
White Circle 54
White Circle 57
White Circle 60 (reprint)
White Circle 62
White Circle 66
White Circle 67