This was apparently a test set, meaning that Topps prototyped – i.e., mocked up – a set of cards devoted to the subject, to get an idea of how the set might perform in the marketplace. That explains why these cards are rather plain, and also why they are very often found blank-backed.
The card fronts show a black and white photo still from the Daniel Boone TV series, surrounded by a white border and with a caption at the bottom. The backs, if not blank, offer a paragraph of text within a rounded border, topped with the card title and the card number at the upper left. Below this is the set title logo and the copyright info at bottom.
Being a test issue, this is a rarely-seen item, with very few apparently having been distributed. It's not known how exactly the existing cards entered the marketplace, given that no wrapper has ever been found; but uncut sheets are known to exist, and the cards cut from these are blank-backed. In fact, the blank-backed cards are about twice as common as the ones with the regular backs.