This is probably the smallest print I have ever seen on something that was designed to convey instructions! If you want half a chance at deciphering the secret behind the trick, feel free to click and enlarge the scan!
The scan on the left is how I found the card tucked into the package. After reading the instructions, I had to imagine seeing this performed, and I have to admit that I think it would have been obvious that the little center panel could slide back and forth. While the scan doesn't really convey it well, the flap that the panel slides through really protrudes from the card.
Yet, one of the interesting things about these tricks is that while I am seeing them sometimes in their most primitive states when they were first introduced to the public, they have evolved and are still used today.
I learned via the Genii Magazine message boards that this trick was designed and sold by DeLand around the turn of the century, and over the years it was engineered with heavier material and became "Giant Monte". You can watch a version of it here. Although it is not in English, you will still get the idea!
Here is a better look at the sliding insert:
Here is the back of the card- I always take note of different DeLand's card designs.