A = Anonymous

Pillar boxes manufactured by Andrew Handyside between 1879 and 1883 omitted the words POST OFFICE and the VR royal cipher, and these boxes are known as Anonymous as a result. When the omissions were pointed out the situation was immediately rectified, but not until Anonymous boxes had been produced for five years.
The Letter Box Study Group
2016-02-10T15:57:16+00:00
Pillar boxes manufactured by Andrew Handyside between 1879 and 1883 omitted the words POST OFFICE and the VR royal cipher, and these boxes are known as Anonymous as a result. When the omissions were pointed out the situation was immediately rectified, but not until Anonymous boxes had been produced for five years.