Try this image as a jigsaw

On the Brighton Road midway between Posts 163 and 164 marking the boundary between:
Reigate and Banstead / Croydon
Highways Authority / Transport for London - this being a trunk road

TQ 3120 6176. Someone sent me this picture and asked what is it? It is painted on a wall in Foxley Lane, Purley near the junction with Russell Hill Road, ie. at the junction with those two roads and the Purley Way and Pampisford Road.
The text is “DOT N^o. A23 1900 R”. DOT is Department of Transport, A23 is the road number and 1900 R is the entry in the DOT’s list. But list of what? The answer is: list of walls, bridges and other structures which were the responsibility of the DOT. (I think I have seen similar signs on bridges.) The name Department of Transport existed between 1976 and 1997. It and its successors are responsible for trunk roads except, as the "HA/TfL" notice in the previous image indicates Transport for London looks after this wall.

But why? At some stage, probably at the time of the creation of the Purley one-way system, this bit of Foxley Lane was widened necessitating a retaining wall which became the responsibility of the DOT.