The Crown was added in 1812 I believe, so the date range should be 1787-1812.I did some looking, and I'm agreeing with the late 18th century. I couldn't find the exact match, but the wreath and anchor without the crown added above the anchor was worn by Royal Flag rank. The crown was added soon after 1800, I believe. Super find! There just weren't that many Admirals walking around back then.
In 1774 a fouled anchor, plain edge or surrounded by rope edging, took the place of the Rose and in 1787, the same design, with the addition of a wreath of laurel leaves, was adopted for the Admirals.