I was speaking with an 88 year old coin dealer earlier today and the subject of 1951 crowns was discussed. This dealer told me that he got married during the Festival of Britain celebrations in London. He explained to me the reasons for the two different colour boxes that the Crowns were issued in, Green & Magenta.
The Green boxes were sold only in the Festival Hall, The Magenta boxes were sold in the Festival Pleasure Gardens on the opposite bank of the Thames.