That is a very good question. The Ketlands probably weren't active exporters in their own right until after 1792. I suspect this was a shipment of guns purchased in England (or elsewhere) and shipped through a neutral port, like Lisbon or Amsterdam, in a British vessel and trans-shipped in an American flag vessel. Arms exports to America were not authorized by the Privy Council until 1792 so some sort of subterfuge probably took place. That is a very large shipment and the prices are quite low which leads me to think they weren't really "new" or that they were very cheap commercial guns. In 1812 the price of the cheapest Ketland export fowler was 17 shillings which I think comes close to the $3.00 these guns were being sold for... so they had to have been purchased for much less than that.
They might also have come from Ireland. I'm not certain what the legal ramifications of that were but Ireland still had its own Parliament in 1785 and was not officially part of "Great Britain" until 1804. Whether Privy Council regulations applied there is a good question for someone more knowledgeable than I am in the complexities of British Government at the time. Interestingly, the Ketlands are known to have exported a large number of muskets to Ireland in 1785... one has to wonder if this was the same shipment. There was a later shipment to Ireland, also a large quantity, that attracted the attention of the government as I have some of the correspondence relating to it.
Were it not that the Boston Customs House burned to the ground in 1842, we could look this shipment up and find out exactly where it came from, who consinged it and who it was shipped to.