  The low [alpha/Fe] ratio in the metal-poor ([Fe/H]= -1.50) common proper
motion pair HD 134439 and HD 134440 has been variously attributed to chemical
evolution in an extragalactic environment with an irregular star formation
history, planetessimal accretion, and formation in an environment with an
unusually high dust-to-gas ratio. We explore these various putative origins
using CNO, Be, Ag, and Eu abundances derived from high-resolution near-UV
Keck/HIRES spectroscopy. While we confirm a previously suggested correlation
between elemental abundance ratios and condensation temperature at the 95%
confidence level, these ratios lie within the continuum of values manifested by
extant dSph data. We argue that the most plausible origin of our stars'
distinctive abundance distribution relative to the Galactic halo field is
formation in an environment chemically dominated by products of Type II SN of
low progenitor mass; such a progenitor mass bias has been previously suggested
as an explanation of low alpha-element ratios of dSph stars. The proper motion
pair's heavy-to-light $n$-capture element ratio, which is > 0.3-0.5 dex lower
than in the Galactic halo field and dSph stars, is discussed in the context of
the truncated r-process, phenomenlogical n-capture production models, and
alpha-rich freezeout in a high neutron excess environment; the latter
simultaneously provides an attractive explanation of the difference in
[Ca,Ti/O,Mg,Si] ratio in HD 134439/134440 compared to in situ dSph stars.
