Before the station was rebuilt there were six full-length platforms, and that was where platform six was. But it's not part of the original one; both the outside platforms were demolished in 1975 so that tracks could be moved outwards and the two island platforms widened.
Obviously that bit was rebuilt, for some reason. Perhaps it was just as a retaining wall for the higher ground with car parking and the fence on it. However, while slam-door stock was still operating there was nothing to really stop people getting off. They just had to be sensible then!
But unloading newspapers from down trains with direct level access to road transport does sound plausible. I'm not sure how long after 1975 use for mail would have been called for.