The story of Edward II's marriage contract to Isabella of France is a fascinating one. It's been held in the archives in Swansea Council/University of Swansea. It's never been on show at any of the museums in Swansea. The story of the document is quite fantastical, I think. It was given to a Doctor in the 19th Century by a farmer in lieu of money for treatment. The mystery is, how did the farmer get it? The story says he has had it in his family for many years, but unable to explain how it came into his family. At long last, Swansea Council has put up an article on the document - click on the links below to read all about it.
You can also download the document and translation. Although the article says Edward sent his goods and documentation to Swansea Castle, it's my understanding they were kept at Neath Abbey, a couple of miles from Swansea Castle. Over the years, many coins from Edward's time have been discovered hidden away. It's also been established that many of his goods were stolen when he fled Neath Abbey.
A photo from the Victorian era which shows the document with seals attached.