Many of the earliest reprints have a feature I've never seen with other mass market publishers - changes to the colouring of the covers. Here's a good example. Wanted for Murder was published as a WC in 1943, the second printing in 1944, the third 1945. Both reprints have the date on the copyright page. The third printing has the price added to the front cover. There are a few differences between the covers. The last two pages are house ads, different for each printing. The address on the title page has changed for the third printing to 70 Bond Street, Toronto from 48 Pall Mall, London.
White Circle 106 first printing
White Circle 106 first printing back cover
White Circle 106 second printing
White circle 106 second printing back cover
White Circle 106 third printing
White Circle 106 third printing back cover