After I spent yesterday evening with referencing postboxes in my area (http://www.dracos.co.uk/play/locating-postboxes) I realised that there were still a lot missing. Then, a bit later that evening I had an idea: Postbox orienteering.
So, today I printed a map of the area and marked the approximate position of all the postboxes which I could not find yesterday evening. Then I set out for my postbox orienteering run. Whenever I found one of the postboxes I marked their exact location the map and jotted down the reference code.
12km later I had 12 new postboxes and finished a very nice run.