I guess it's down to interpretation. From an LBSG post of view it's lamp box design, squatter than a wall box and with slightly curved corners on front face. There are lots of mostly rural lamp boxes that have been embedded into walls and wall mounted lamp boxes are mentioned in the guidance at https://wiki.openstreetmap.org/wiki/Key:post_box:type. They're presumably cheaper to make than wall boxes and if they're in rural location don't need the capacity of even the modestly larger type c wall box. There are quite a few examples at https://www.flickr.com/groups/1074749@N21/pool/ including my favourite at Durgan. I then follow guidance at post_box:mounting to state if the box is embedded in a wall or not. But there are also a few wall boxes that aren't embedded into walls (e.g.  Cowley Bridge, Exeter EX4 251 near me (https://en.wikipedia.org/wiki/File:Exeter_post_mounted_wall_box.jpg) which is definitely a type b wall box) which I must get around to adding to OSM. I'll happily go either way but at present follow https://wiki.openstreetmap.org/wiki/Key:post_box:type