Had this yesterday. It's a cut coin, the full coin would have been large and thick. Looks like it could be a famous piece of eight . Same session also had another very rare Yorkshire 17th Century traders token.
Well its not as early as the cob I wish for, but still a real different find for a landlubber....I would put a half-cross on my list, but it would be added to the 'firsts'. Cool find.