From Tracy Simpson's
U.S.Postal Markings 1851-1861 (revised 1979):

One of the larger size markings as yours is. Note that it's only struck once on a cover. And the cover above has an incorrect use besides; should have been a 12c credit to Prussia.
There is another Boston type not shown in the style of the NY ones given by
John Becker, but yours definitely seems to have a "K" next to the "7" rather than an "N".