Find the dissimilar number. 216, 343, 504, 512, 729

Let's analyze the numbers in the given set:
* $216=6^3$ (Perfect cube)
* $343=7^3$ (Perfect cube)
* $512=8^3$ (Perfect cube)
* $729=9^3$ (Perfect cube)
* $504$ is not a perfect cube.
Therefore, 504 is the dissimilar number.