This effect comes from a Phil Goldstein (Max Maven) effect published in the March 1980 "Peter Warlok’s The New Pentagram" tilted "Surprise By The Numbers". It is a humorous revelation of a selected number by a spectator.
The magician comes forward with five cards that have the numbers 1 through 5 printed individually on the faces. He hands the cards to a spectator and has them shuffled. The magician takes the cards back and has the spectator pick one of the cards, remember it and put it back anywhere they want, in the pile of the four remaining cards.
The magician picks up an envelope and shows it empty. He then places the cards numbered one through five in the empty envelope, closes the envelope, and sets it on the table. The spectator has had complete control over the entire proceeding, however the magician explains that he will be able to tell which card the spectator selected by only the feel of the card.
The magician reaches into the envelope and pulls the cards out one at a time with out looking, and places them face down on the table. He stops at the fifth card and tells the spectator that this is the card they selected.
When the magician turns the card over, it is revealed to be the number 4. The spectator exclaims that this is not their card! The magician asks the name of the chosen card. The spectator proudly states that it was a 3! The magician turns the 4 around so from the spectators view it looks like an "h"… and slowly turns over the four cards one at a time revealing the letters T...R...E…E instead of the number 1,2,3 and 5!
The magician has left a space between the T and the R, reaches down and fills the space with the upside down 4 spelling…...THREE.
If you like the plot have fun building the effect and remember "Magic with out misdirection is not magic" - Slydini.