  We have carried out polarization calibration for archival JVLA ($\sim$9 mm)
full polarization observations towards the Class 0 young stellar object (YSO)
OMC-3/MMS 6 (also known as HOPS-87), and then compared with the archival ALMA
1.2 mm observations. We found that the innermost $\sim$100 au region of
OMC-3/MMS 6 is likely very optically thick (e.g., $\tau\gg$1) at $\sim$1 mm
wavelength such that the dominant polarization mechanism is dichroic
extinction. It is marginally optically thin (e.g., $\tau\lesssim$1) at $\sim$9
mm wavelength such that the JVLA observations can directly probe the linearly
polarized emission from non-spherical dust. Assuming that the projected long
axis of dust grains is aligned perpendicular to magnetic field (B-field) lines,
we propose that the overall B-field topology resembles an hourglass shape,
while this "hourglass" appears $\sim$40$^{\circ}$ inclined with respect to the
previously reported outflow axis. The geometry of this system is consistent
with a magnetically regulated dense (pseudo-)disk. Based on the observed 29.45
GHz flux density and assuming a dust absorption opacity
$\kappa^{abs}_{29.45\,GHz}=$0.0096 cm$^{2} $g$^{-1}$, the derived overall dust
mass within a $\sim$43 au radius is $\sim$14000 $M_{\oplus}$. From this case
study, it appears to us that some previous 9 mm surveys towards Class 0/I YSOs
might have systematically underestimated dust masses by one order of magnitude,
owing to that they assumed the too high dust absorption opacity ($\sim$0.1
cm$^{2}$ g$^{-1}$) for $\sim$9 mm wavelengths but without self-consistently
considering the dust scattering opacity.
