There are few cards that I am aware of that were so widely licensed as the 1976-77 MSA discs. They are often referred to by the "issuing" company rather than MSA itself, which makes collecting them even harder than the usual card issues of the time.
On the front, most MSA cards look very similar. They are round - about the size of most coasters. They feature a black-and-white photo of the featured player, but the team logos are airbrushed out due to licensing issues with the MLB. The discs feature 'stitches' to make them look like baseballs. The left side of the ball features bio info and the right has the birthplace and copyright info. Under the picture, we see the player's name, position, and team. We also see the MLB emblem for the player's association (which explains the airbrushing). The tops of the card on front is where life starts to get interesting...
As you see in the pictures below, some companies simply used stars, while others actually printed the name of the 'issuing' company (such as Crane potato chips).
The backs of these cards takes collecting to a whole new level of obsession and insanity. Crane features their logo and company name while others have their own logos and info on the backs. Of course, some don't have anything back there at all (hence, they are referred to as "blank backs"). As you see above, I have three Frank Robinson variations, and this by no means encompasses every possible MSA Robinson disc. In fact, this probably doesn't even scratch the surface.
I am very partial to the Isaly's card because we actually had an Isaly's near where I grew up as a kid in Western PA. For the record, they had the BEST chipped ham in the world. If you have never had chipped ham (also called chipped-chop or chipped-chopped ham), you are seriously missing out on a delicacy! Put it on your list of things to eat should your travels ever take you to Pittsburgh or close by outlying areas.
If you plan on adding some of these to your collection, be sure to search for them by their issuing company (Crane, Isaly or Isaly's, Caesars, etc) and also search for MSA Disc. They are generally a bargain, and are often sold in lots. Happy Hunting!