Comments on orientifolds without vector structure

We revisit type I compactifications with a Spin( 32)/Z(2) gauge bundle that admits no vector structure. We elucidate the relation of this Z(2) 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