I have seen a few Mengler Baseball's Best "postcards" online over the past two years, but never stopped to find out what they were until today. A few short Google searches later I had some answers and figured I would share them:
According to the 2010 Standard Catalog of Baseball Cards the set was one of the first card sets to have production stopped by the threat of legal action from the Major League Players Association. Before the plug was pulled Clarence Mengler created a five series, 125-card set of 3"x5" index cards meant to be used as an autograph medium. There are six Phillies included in the set (complete Phillies checklist below).The card fronts feature a drawing by Keith Peterson and the player's name in block type print. Backs are blank, though some feature a red stamped "Mengler-Peterson 1975". The printing on the back is inconsistent, and I have not found a definitive reason why some have it, and others do not.
A source from a collecting message board claims to have spoken with Mengler about the set. According to him, Mengler said he "probably sold 25 complete sets. I think around 200 or so were printed for the first three series, and around 100 of the final two." The thread also notes that the cards came back into circulation after Mengler started selling his inventory online. Despite the relative scarcity of the cards in the set, value for most hover around a dollar or two. You won't make a lot of money collecting them, but you may get some cards your fellow collectors do not have!
Five, 20-card Series' (A-E) - 125 cards in the complete set
Note: Cards are unnumbered, but listed in alphabetical order
Phillies Set Checklist:
#20 Mike Schmidt
#27 Larry Bowa
#28 Steve Carlton
#59 Greg Luzinski
#91 Tug McGraw
#104 Bob Boone