BV is right that it could be as 'new' as early 19th century, possibly British. I would place it between mid-18th to first decade of 19th. As far as cleaning it, everyone has different opinions on that. Because this piece is already in a deep state of corrosion (stabilized), perhaps just oiling it? If you really want the brown rust gone, perhaps steel wool or fine grip sandpaper with oil. I would be afraid the wire brush might clean away the dark patina and leave a 'shiny area' unpleasing to the eye-
Here's a British swivel, circa War of 1812. Note the elevated block surrounding the firing hole, similar to yours...