Finite $p$-Groups of Nilpotency Class $3$ with Two Conjugacy Class Sizes

It is proved that, for a prime $p>2$ and integer $n\ge 1$, finite $p$-groups of nilpotency class $3$ and having only two conjugacy class sizes $1$ and $p^n$ exist if and only if $n$ is even; moreover, for a given even positive integer, such a group is unique up to isoclinism (in the sense of Philip Hall).