This is a very desirable but confusing set for a lot of collectors, and with good reason. Are there 44 cards in the set, or 66? Was there a test release? Do blank-backed ones exist? Here's the most straightforward information that we've been able to come up with.
First, let's examine what all cards in this series have in common. Each one shows a black-and-white still photo from the Superman TV show, surrounded by a white border and with teh card caption in a long oval near bottom center. The backs give the card number within a Superman 'shield' symbol at top left, while the caption is repeated at top center. Below this is a large block of text explaining the scene or episode. A large cut-out photograph of Superman stands at right. Crds numbered 45 through 66 have puzzle backs.
The more common types of cards have dull orange around the text box in the center on back, for cards #1-44. At the bottom of these is either a copyright line (type 1), or the ad 'Watch Superman on TV' (type 2).
The third type has the copyright line, but no color at all on the back. These are considered proof cards - cards which were initially printed to check the printing process, but which didn't quite go through the entire production procedure - which have made their way out into the hobby.
These white-backed Superman cards are extremely difficult to obtain, more so than many of the difficult Topps test sets that collectors vie for.
Note: These cards were available in 5-cent packs containing no gum, although a single wrapper indicating the inclusion of gum has been found. If you find a 1966 Superman pack that mentions containing gum - buy it!