This is coolbert:
"letters of marque [märk]: noun plural - - A document issued by a government in a time of war allowing private ships to attack the ships and seize the property and citizens of a hostile nation."
"A letter of marque and reprisal . . . was a government license in the Age of Sail that authorized a private person, known as a privateer or corsair, to attack and capture vessels of a nation at war with the issuer. Once captured, the privateers could bring the case of that prize [and any booty from the captured vessel] before their own admiralty court for condemnation and transfer of ownership to the privateer"
Basically piracy but during a time of war a legalized and recognized valid action. Again, only during a time of war and then acceptable only permissible when authorized by legally constituted authority.
Described as venerable but with a relatively recent history?
"In December 1941 and the first months of 1942, Goodyear commercial L class blimp Resolute operating out of Moffett Field in Sunnyvale, California, flew anti-submarine patrols. As the civilian crew was armed with a rifle, a persistent misconception arose that this made the ship a privateer and that she and sister commercial blimps were operated under letters of marque until the Navy took over operation. Without congressional authorization, the Navy would not have been able to legally issue any letters of marque."
Commercial blimps operating out of a Naval Air Station [Moffett Field] with an armed crew member referred to [?] as privateers. NO! Not so. Without that Letter of Marque lawfulness highly questionable.