The Red Heart set is one of my favorite sets. This was the first PSA Registry set that I ever did. See link below. There are 11 of each color background, red, blue and green. The red is said to be the toughest one to find. The backs are all the same color. There are only 3 PSA 10 Red Heart cards, 2 Mantles and a Sherman Lollar. As was mentioned, they were supposedly available until the early 1970s as mail in redemptions. Here are scans of each background color and a back scan so you car see what they look like.
James
My set:
https://www.psacard.com/psasetregist...shedset/203939