  Haystack and Owens Valley Radio Observatory (OVRO) observations recently
revealed strong sinusoidal total flux density variations that maintained
coherence between 1975 and 2021 in the blazar PKS 2131-021 ($z=1.283)$. This
was interpreted as possible evidence of a supermassive black hole binary
(SMBHB). Extended observations through 2023 show coherence over 47.9~years,
with an observed period $P_\textrm{15 GHz}=(1739.3 \pm 1.2) \, {\rm days}$. We
reject, with $p$-value = $5.3 \times 10^{-7}$, the hypothesis that the
variations are due to random fluctuations in the red noise tail of the power
spectral density. There is clearly a constant-period physical phenomenon in PKS
2131-021 producing coherent intermittent sinusoidal flux density variations. We
find the coherent sinusoidal intensity variations extend from below 2.7 GHz to
optical frequencies, from which we derive an observed period
$P_\textrm{optical}=(1764 \pm 36)$ days. Across this broad frequency range
there is a monotonic phase shift in the sinusoidal variations with frequency.
The same coherent periodicity is possibly also observed at $\gamma$-ray
energies. The importance of well-vetted SMBHB candidates to searches for
gravitational waves is pointed out. We estimate the fraction of blazars that
are SMBHB candidates to be $>1$ in 100. Thus monitoring programs covering tens
of thousands of blazars could discover hundreds of SMBHB candidates.
