I haven't seen much about them in a quick search. There are only 2 Ebay auctions I could find one was a 26 card lot of the Stan Musial insert mentioning he had 20 different cards MM1 - MM20 so there is a minimum of 20 Stans. The other auction was for a 74 card lot but doesnt' state how many different cards.
I collect: Baseball, Football, Hockey, Mostly Vintage pre1980, My Homie teams - Washington/Baltimore Teams Senators (Twins, Rangers), Expos/Nationals, Redskins, Capitals, Bullets/Wizards - HOFers - Non-sport (mostly TV shows and movies).
My Trade List is very much a work in progress
CaptKirk42s Trading Card Blog Curly W Cards Strive For '65 YouTube klandersen42