It appears to have the both the overprint and underprint "O.U.S" of the Oxford Union Society but I don't see why it should have both.
The stamp appears to be plate 113 and the 603 postmark is from Oxford.
The OUS is the only organisation listed in the SG Specialised GB Victoria catalogue to have used its own private overprint.
This overprint was suppressed in 1869 but it was possible in the years 1867-1882 to have a private underprint (i.e. an overprint on the back of the stamp) applied to sheets before the sheets were gummed. Only five organisations including the OUS ever took up this service.
