Nice find! Underwood made inks before Thomas Underwood embarked on his more well known venture—typewriter manufacturing—in 1895. The company continued to make both inks and typewriters concurrently into the early 20thC. I agree with Jim on the age... 1880s–1900s.
Thanks, guys! The river I'm finding these things in has been populated by europeans since the 1600's and had a ferry line from NYC for years, paddle wheel steamers. Every time we fish that area we find bottles, I suppose it had a lot to do with the fact that all the trash went right over the side into the water back then. I can imagine a clerk on the ferry finishing up his daily paperwork onboard, running out of ink and flinging that little bottle out a porthole lol