Non-public boxes emptied by Royal Mail would not have to carry a cypher. Royal Mail article boxes made considering that 1994 carry the wording 'Royal Mail', Commonly over the aperture (lamp boxes) or on the doorway (pillar boxes). Before this date all publish boxes, aside from the Nameless pillar boxes, https://ok-social.com/story13830418/letter-boxes-for-sale-no-further-a-mystery