Today we have what looks like a normal US to UK letter paying the 24 cent rate in 1867. The light New York 3 Paid marking indicates that it was sent on an American packet, so 3 cents were passed to the British post office and the remaining 19 cents were kept by the United States to pay for the 16 cent Atlantic voyage and five cents for US surface mail.
The interesting part of this is the "R" in a "ring" marking. This mark indicated that the letter was forwarded to a new address for free because it did not leave the postal district.
This is the only example of the R in ring I have found with a 24 cent stamp, but I wonder if there might be others.
If you would like to read more, I do have a blog with much more detail.