  I report a tentative ($\sim4\sigma$) emission line at $\nu=100.84\,$GHz from
"COS-3mm-1'", a 3mm-selected galaxy reported by Williams et al. 2019 that is
undetected at optical and near infrared wavelengths. The line was found in the
ALMA Science Archive after re-processing ALMA band 3 observations targeting a
different source. Assuming the line corresponds to the $\rm CO(6\to5)$
transition, this tentative detection implies a spectroscopic redshift of
$z=5.857$, in agreement with the galaxy's redshift constraints from
multi-wavelength photometry. This would make this object the highest redshift
3mm-selected galaxy and one of the highest redshift dusty star-forming galaxies
known to-date. Here, I report the characteristics of this tentative detection
and the physical properties that can be inferred assuming the line is real.
Finally, I advocate for follow-up observations to corroborate this
identification and to confirm the high-redshift nature of this optically-dark
dusty star-forming galaxy.
