All parts have writing on them. (See image below) Do you mean that the centre part which carries the details was not visible from the road?
If so, that is actually contrary to the Department for Transport's authorisation* of the Hounslow suspension sign. It is a condition of the authorisation that the details of times, dates and location should appear on the FRONT of the sign. (In the official diagram, the centre part is marked "Front Panel" and the outer parts as "Side Panels".)
If it wasn't displayed like that, it would be worth a challenge claiming that the sign was not displayed in accordance with para 1(a) of DfT authorisation GT50/086/0002 dated 15 December 2011.
* http://assets.dft.gov.uk/trafficauths/case-2623.pdf