Sasaferrato
Super Refuser!
Our guide showed us a few of the penny blacks in the library, which are all on show to the public. She dismissed them as not being particularly valuable. They have many stamps available for viewing.
Depends what you call valuable.
As you would expect, or perhaps you wouldn't, with 68m stamps printed, the printing plates wear out. There were 11 PB plates (12 if you count 1a and 1b), plate 11 is the rarest. a mint never hinged PB Plate 11 catalogues at £18,000.00.
Of course, if you come across a British Guyana 2c black on magenta, you can add another three zeroes to that.