They were made for Wiffle Ball.
The discs came on the Wiffle Ball box. Though they looked just like all the other MSA discs, the Wiffle discs were smaller; their diameter was 2 3/8" which is an inch smaller that the others.
The dating of the discs is a bit confusing, too. The copyright on them is 1976. But that doesn't jive with this one...
...of Bill Buckner. He came to the Cubs in a trade with the Dodgers on January 11, 1977. Unless the good folks at MSA had a crystal ball, the date on the Wiffle discs is wrong. The Standard Catalog lists the release date as 1977, not 1976. I wonder what gives with all the dating confusion.
Two other Cubs were in the set...
....Manny Trillo and Ray Burris.
Here's another oddity with the Wiffle set. The three Cubs in the set were not included in any of the food disc sets released at the same time. The Cubs in the '76 food sets were Jose Cardenal, Bill Madlock, Rick Monday, Jerry Morales, and Andy Thornton. The next year saw Jose and Bobby Murcer in the food set; no Billy Buck, Burris, or Trillo. Plenty of players from other teams were in both Wiffle and food sets, but not the Cubs.