Was an experiment to provide additional ventilation in the engine room. '456 had the glass bodyside windows at one end replaced with grills in Jan 1985 and '542 had the same mods in the April. They went further with 542 and changed the glass windows at the other end then also did the bodyside door grills (1986-88). Reference 'Class 47:50 year history' (Lilley/Hiscock & Ward).
Oops: ExRes beat me to it![]()