  The nature of the peculiar `Ca-rich' SN 2019ehk in the nearby galaxy M100
remains unclear. Its origin has been debated as either a stripped core-collapse
supernova or a thermonuclear helium detonation event. Here, we present very
late-time photometry of the transient obtained with the Keck I telescope at
$\approx280$ days from peak light. Using the photometry to perform accurate
flux calibration of a contemporaneous nebular phase spectrum, we measure an [O
I] luminosity of $(0.19-1.08)\times10^{38}$ erg s$^{-1}$ and [Ca II] luminosity
of $(2.7-15.6)\times10^{38}$ erg s$^{-1}$ over the range of the uncertain
extinction along the line of sight. We use these measurements to derive lower
limits on the synthesized oxygen mass of $\approx0.004-0.069$ M$_\odot$. The
oxygen mass is a sensitive tracer of the progenitor mass for core-collapse
supernovae, and our estimate is consistent with explosions of very low mass CO
cores of $1.45-1.5$ M$_\odot$, corresponding to He core masses of
$\approx1.8-2.0$ M$_\odot$. We present high quality peak light optical spectra
of the transient and highlight features of hydrogen in both the early (`flash')
and photospheric phase spectra, that suggest the presence of $\gtrsim0.02$
M$_\odot$ of hydrogen in the progenitor at the time of explosion. The presence
of H, together with the large [Ca II]/[O I] ratio ($\approx10-15$) in the
nebular phase is consistent with SN 2019ehk being a Type IIb core-collapse
supernova from a stripped low mass ($\approx9-9.5$ M$_\odot$) progenitor,
similar to the Ca-rich SN IIb iPTF 15eqv. These results provide evidence for a
likely class of `Ca-rich' core-collapse supernovae from stripped low mass
progenitors in star forming environments, distinct from the thermonuclear
Ca-rich gap transients in old environments.
