Is This a Wreck Cover?
As I suspected, it is only a front, but it has a tantalising partial postmark on the remnant of flap attached. It's a partial "Liverpool" postmark in blue, which I would expect to be a "Liverpool Packet" postmark. However, there doesn't seem to be a matching date for that postmark, according to "Collect British Postmarks" (Stanley Gibbons, 8th Edition), bearing in mind that the Lochmaben sank in 1855.
Unless I can find a passenger manifest for the last voyage of the Lochmaben to see if Mrs. Hazelton was aboard, I will probably never know if this is a wreck cover!