So I've embarked upon my mission in earnest. I now know that the print I'm looking for is entitled "Bub and Sis" from the set "In Our Time," and the artist is R.B. Kitaj. Unfortunately, I'm also finding out that this might be the one thing in the world that is not for sale on the Internet. Just my luck. I'm going to keep looking, but if anyone has any rare-print-art-dealer connections or superior internet-research skills, I would be forever grateful.