I personally don't subscribe to the baggage issue theory. The Queen descended the steps of the plane on arrival wearing one of the brooches, and if some baggage had been left behind I think it would have been put on the next flight out. I don't think the Queen would borrow lots of jewels (the exception of a tiara from Lord Plunket was very last minute). Leslie Field in an article recalls how she had to remove the word 'borrowed' from a draft of her book as the Palace said the Queen doesn't borrow!

I think the most likely explanation is that, as is her custom, these jewels were gifts from Nigeria and maybe some other neighbouring African Commonwealth countries. One may well have been given as a gift during that trip. They haven't been worn at any other time as it may be a bit embarrassing being given jewels from a country with much poverty. All of the brooches I like but the Queen has so many others to choose from. One, and maybe two, of the diamond brooches look like they could be modelled on African flowers? The other jewels such as necklaces and earrings I don't like and the Queen has much better looking ones to choose from for other occasions.

Whatever the true explanation is, it was certainly an odd trip from the jewels perspective!