I have a vague recollection of reading somewhere that a Japanese company made pencils in half-grades: I must check.
Edited: The above is almost, but not quite entirely, incorrect. F/2.5 is the only half grade.
Meanwhile, please talk among yourselves, as I may be some time, and read The Pencil by Henry Petrosky (honest! It's a brilliant history, written by a Professor of engineering. He also wrote The Evolution of Useful Objects* and To Engineer Is Human).
* Which included The Paperclip... new thread, anyone?