It's a handsome set. They were available via a mail in order and I have heard that the company had sets they would send up until the 1970s. The backs are one color, but have a good design and offer information a fan would like to know.
Issued in 1955, they are an excellent supplement to mainstream sets as they include both Mantle and Musial. Mantle did not appear in the Topps set and Musial didn't appear in the Topps or Bowman set. Here's a link that has a picture for the card back. They are a relatively common but desirable issue.
https://keymancollectibles.com/baseb...54redheart.htm