  We present new observations of the gravitational lens (GL) system B1600+434,
strongly suggesting that the lens is an edge-on spiral galaxy. These
observations are used to constrain the mass model of the system, in particular
the oblateness and velocity dispersion of the dark matter halo around the
lensing galaxy. From an analytical model we find a lower limit on the halo
oblateness $q_\rmn{halo}=(c/a)_{\rho}\ga0.4$; more detailed numerical models
give a lower limit $q_\rmn{h}\ga0.5$. We determine an average halo velocity
dispersion of $\sigma_{halo}=190\pm 15$ km/s over all non-singular isothermal
elliptical (NIE) halo models. Constraining the models to larger and more
massive disks, decreases this average by only 10 km/s. A lower limit of
$\sigma_{halo}\ga 150$ km/s is found, even for disk masses larger then the mass
inside the Einstein radius. This lower limit indicates the need for a massive
dark matter halo, contributing at least half of the mass inside the Einstein
radius. Time delay calculations give $(54\pm3)/h_{50}$ days for the NIE halo
model and $(70\pm4)/h_{50}$ days for the modified Hubble profile (MHP) halo
model. Although the time delay for both NIE and MHP halo models is well
constrained on our parameter grid, it strongly depends on the halo surface
density profile. We furthermore find that the presence of a flat luminous mass
distribution can severely alter the statistical properties of the lens.
