The first city William of Orange entered was Exeter. I don't know if that's of much help. I've not heard of any such claim I'm afraid and where I live is classed as the West (just).
The sort of document you talk of may be the kind of thing held in cathedrals. Try the cathedrals first and see what they have in their archives. Off the top of my head Salisbury, Winchester, Wells and Exeter jump out.
If not a cathedral then a town hall, but if it's a town and not a city you've got your work cut out. There's a lot of towns out there.
This is a link to Exeter Cathedral's archives to help start you off. They might even be able to answer your question.