Your cover is quite normal.
The wording is a bit unclear, but from section 890.3 of the 1913 "Postal Laws and Reglations:
Registration regulations evolved over time, so when your cover was sent, the dated postmarks are properly used on the flaps on the reverse side and the stamps obliterated with some other handy non-dated device. This was often something used on 3d class or parcel post mail. In your case the straight-line rubber stamp is more the type used to fill-in-the-blank on various forms, rather than used to cancel stamps, but it accomplished the job within the regulations. You will find all sorts of non-dated rubber stamps to cancel the stamps on registered mail. (And you will occasionally find dated cancels on the front too.)