Robert Whittaker
I would treat the D as part of the reference number and include it in the ref=* tag on OSM if that's what's displayed on the collection plate. It indicates "collection on delivery" where the postman/woman empties the box as part of their delivery round.

My comparison tool at https://osm.mathmos.net/postboxes/ is aware of possible D suffices, and will match refs both the with and without the D to the underlying box number. (The same goes for P suffices on parcel boxes.)