The Keisler–Shelah isomorphism theorem and the continuum hypothesis

We show that if for any two elementary equivalent structures $\mathbf M, \mathbf N$ of size at most continuum in a countable language, $\mathbf M^{\omega }/ \mathcal U \simeq \mathbf N^\omega / \mathcal U$ for some ultrafilter $\mathcal U$ on $\omega ,$ t