directly for guano, The British had a trade monopoly with Peru for it's guano, the US tried to get a direct treaty to trade but was unsuccessful, so they passed the Guano Island Act of 1856 which allowed US citizens to annex any guano producing island unclaimed by other countries as US territory, I think about 100 islands were annexed under this act. Otherwise we would have to buy mostly expensive guano from the British, indirect trade with Peru for Guano rapidly diminished after the 1856 Act.

My guess is Anderson might be making a reference to a William R. Grace, who began his career importing guano from Peru during the early 1850's, by suppling Peru's miners with cheap equipment in exchange, so he could bypass the monopoly. He later became Mayor of New York, so would be yet another fairly famous person our protagonist is associated with.