  We revisit type I compactifications with a Spin(32)/Z2 gauge bundle that
admits no vector structure. We elucidate the relation of this Z2 obstruction to
discrete B-field flux and to 't Hooft flux and clarify some subtleties in the
T-duality transformation to type IIA intersecting D-brane models. We reexamine
the earliest 3-generation GUT model on magnetized D-branes and show its
consistency when a discrete B-flux is switched on. We further generalize
partially known results for toroidal models to type I compactifications without
vector structure and their mirror dual type IIA orientifolds on genuine
Calabi-Yau manifolds. We illustrate this by working out the example of the
quintic in some detail.
