Wien (Vienna) rectangle registered mail cancellation. (26/7/1864).
Here`s an example:
http://www.austrianphilately.com/wctrc/index.htm(scroll down, 7th picture). I don`t know if it`s rare, you should look for Edwin Müller catalogue.
Unfortunately I don`t have it.
Sometimes ink colour affects CV (usually higher, if ink is red, blue or green)