Automatically repositionable output stack retention system

In a sheet ejection and stacking system, such as for a document handler output, in which an upper sheet feeding and ejection unit with a sheet ejection path is repositionable between an open and a closed position relative to a lower sheet stacking unit which is providing a sheet stacking tray for the ejected sheets, here a platen cover unit. The sheet stacking tray has an automatically adjusting sheet stacking registration edge wall defining system which is automatically maintained closely adjacent to the sheet ejection position of the upper unit. This system is defined by upstanding stack retaining wall members movably mounted to the sheet stacking tray and a plurality of aligning post members integral the upper unit which automatically engage and move the stack retaining members into alignment with the upper unit, into a proper sheet receiving position therewith, when the upper unit is closed. Yet, the retaining members retain the ejected sheets substantially in their desired stacking position when the upper unit is opened. The aligning post members overlap and mate within the stack retaining members in the closed position to fully block movement of ejected sheets into or under the upper unit. Considerable mounting or other misalignment tolerances are automatically compensated for between the two units with this system.