Check ref: RM 2013 data suggests this should be HP20 77 (see note for date of most recent check that confirmed the apparently erroneous HP20 71)
note
The reference (HP20 71) is a duplicate. According to RM list, it should be HP20 77 but survey shows same information plate as "real" HP20 77. Can only assume RM have made a mistake and stuck the wrong information plate on this postbox.