The diamond plates were not always put back onto the locomotive after a refit. Some were taken for souvenirs.
These plates were special, very recognisable. I have one at home I was given as a gift.
Does the North British Locomotive diamond plate mean anything to you? North British locos used to be exported all over the world - have you ever spotted an NBL plate in another country?
It was in 1908 when they changed to the diamond shape for the plate. Before that there were three shapes for plates, one for each company that formed North British Locomotive. The diamond became famous around the world.
NBL built good locomotives. The workmanship was very good. The works plates, the diamond ones, were sometimes removed when the cab parts were changed. But you still knew it was a NBL locomotive.