Gels of a poly(oxyethylene)-poly(oxybutylene) diblock copolymer in an aqueous solution forming a bcc phase have been examined using small-angle neutron scattering (SANS) with simultaneous rheology. Application of large amplitude oscillatory shear was observed to induce macroscopic alignment of twinned bcc crystals. The structure of these crystals was probed in three dimensions using SANS with a shear-oriented sample that was fixed in situ in the shear cell and mounted on a goniometer. This corresponds to a kind of "mesoscopic crystallography" which enabled the eludication of the twinned bcc structure, in which the [111] direction was oriented along the shear direction, this being a 6-fold rotation axis. Flow was found to occur in (110) planes oriented normal to the shear gradient direction. A comparison is made between this twinned structure, probed comprehensively in three dimensions, and those previously reported for block copolymer solutions and melts.