Nice work Andy. The cross hairs could be mounted at the rear posotion, but this was normally for ground AA use, when a butt-stock was fitted. This was due to the different eye relief of the gunner. Front mounting is correct for air-to-air use, where, depending on angle, for example in an extreme oblique shot the rear post sight could be ignored, relying on the cross hairs to frame or 'lead' the target.