A short letter of distinction (4)
I believe the answer is:
I believe this is a double definition.
'a short letter' is the first definition.
(I know that billet can be written as note)
'distinction' is the second definition.
(I know that distinction can be written as note)
'of' is the link.
(Other definitions for note that I've seen before include "A piece of paper money" , "Comment" , "Tenner, for example" , "Memo; piano key" , "Mark - sound - money" .)