Tracer, I believe they were issued by the Feds.
In The Wine Dark Sea, Aubrey has captured a privateer w/o papers:
‘So I understand. But wars are conducted according to certain forms. They are not wild riots in which anyone may join and seize whatever he can overpower; and I fear that if you can produce nothing better than the recollection of a letter wishing you every happiness you must be hanged as a pirate.’