D = Dual aperture

Dual aperture boxes first appeared in 1899. Large oval boxes designated as Type C, they provided apertures into separate halves of the box. Originally introduced in London to help pre-sort mail into “LONDON and ABROAD” and “COUNTRY” destinations. In the 1960s Type C boxes replaced all the single aperture boxes in central London, sweeping away many early and historic boxes.
The Letter Box Study Group
2016-02-10T16:05:14+00:00
Dual aperture boxes first appeared in 1899. Large oval boxes designated as Type C, they provided apertures into separate halves of the box. Originally introduced in London to help pre-sort mail into “LONDON and ABROAD” and “COUNTRY” destinations. In the 1960s Type C boxes replaced all the single aperture boxes in central London, sweeping away many early and historic boxes.