Legal Historians: Help me with this Blackstone footnote.

I’m browsing Blackstone and he frequently cites to something identified only as “F.N.B.”, followed by what I assume are pages numbers.

Anyone have any idea what “F.N.B.” refers to?

Fitzherbert’s Natura Brevium: http://www.law.harvard.edu/library/collections/special/sixteenth_century_english_lawyer.php

Gfactor has it.

Aha! that’s it! Thanks, guys.