I was so focused on Pinkerton's that I neglected a clue in the very last line of the letter where it is indicated that the agent was in communication with the U. S. Consul at Barranquilla. I realized that the National Archives may have records of the consul which may reference that correspondence.
That's on my list and I've already contacted a researcher to see what they can ascertain. Hopefully I'll either hear from Pinkerton's or be able to locate these records.
Lesson--actually a reminderDon't get so focused on one aspect of something that you neglect other clues that could be just as helpful.