I do not really have much to add, although I was curious about the name "Charles A. Groth" that appeared on the stamp.
In my research, I found this rather obscure news article about a minor assault incident occurring on the NY "El" (elevated train, for those who may not know) back in 1894. It mentions a Charles A. Groth as being an "Express Agent" with an address of 225 Greene Street, New York.
It wouldn't be out of the question that this may have been the same Charles A. Groth referenced on the stamp. (The occupation, date and location would seem to fit.)
In any event, if nothing else the article gives you an interesting look into a rather foolish encounter dating back 120 years ago:
