I must be in a preview screening mood this week.
At first glance this looks like a 1951 Fan Card - and it is. But it is also an invitation for a preview screening at the Walt Disney Studio Theatre in advanced of the 1974 re-release - the first time the movie had been released domestically since its original release in 1951!
In the 1950s, the studio printed up about a gazillion of these fan cards for pretty much every film they released. A fan card is simply an 8x10 card-stock picture of a scene, character, or art from the film. To get one of these fan cards, you simply wrote the Disney studio a letter, telling them how much you enjoyed the movie, or whatever. They would then send you this card. Pretty cool. I know for a fact that they continued this practice at least through The Little Mermaid (I sent in a letter myself). Problem was, they had WAY more of these than they needed. Disney was even selling originals for a time at WDW MGM Studios at Sid Cahuenga's in the 1990s.
But, back to the topic at hand. Since they had SO many of these left over, they decided to put them to good use and printed up the invitation to the special screening on the back. Apparently they must have realized that people would want to keep these, so they printed right on the card that you could keep it as a souvenir, and that the mailing envelope was the ticket.
A 3 month preview is pretty far in advance, one can only guess why they chose to preview it so far in advance of the release.