Having been in contact with the individual who currently owns the rights to and
all remaining stock of the the outstanding Gamelords material, both for
Traveller and a fantasy setting that shames any on the market today, I can say
with authority (having seen the list of every item in his store of misc items)
that _Letter_of_Marque was never published. Gamelords sold out unexpectedly
with some items still in line for release... which subsequently never were. I
assumed this is one of those ill-fated books.
Hope this helped a bit.