Oooo Arrr, maybe this be it, then?
http://cjoint.com/data/gwqaxIVTpb.htm



"Sir F. A. Rien"
Guinness say the oldest are two at Hauteville and Union streets, St. Peter
Port, Guernsey. Erected 8 February 1853and never changed out.

Confirmed by Jean Farrugia, Archivist of the B.P.O. Guernsey issued a
stamp
in their commemoration.

So, unless removed since the Guinness book was published, they be it.