To be honest I don't know when it showed up, but I do know that sometimes it's left out, either because people forget it or because they don't realize it's importance if you want to run a breather, PCV or whatever out of the rear opening. It won't fit without major cutting when running roller lifters, and if you run solid rollers with pressure oiling to the rollers (such as Crower HIPPOs), that
oil comes from the same passages the hydraulic rollers get their
oil, so you've got to cap the rear intake hole when running those too.
Dan