Here is a link to some information on the Old Cardboard site about the set. It includes a set checklist and images of some of the cards.
http://www.oldcardboard.com/r/r312/r...?cardsetID=869
The R312s are a premium. They are printed on thin paper making them more like a paper pin up picture than a cardboard card. They are not rare but not highly regarded or popular which can make tracking down a specific example a little difficult. If you follow R312 in Cards on ebay you can pick up any that show up that you like. Depending on player and condition you can pick up most that come up for auction for less than $50. I am no expert but I appreciate the affordability of these premiums compared to the more sought after cards of the era.