Not sure but there's a very good chance it's Caruso. I know Caruso has made RL's Black Label suits in years past and also made the blazers and dress pants for the old Rugby line (which was a great line, by the way, far superior to Denim and Supply).
Try a quick Google search for "ralph lauren" and "caruso," there are multiple hits across a number of clothing forums.
Edit: found this old SF thread that describes the suits of the various RL lines. First post mentions RRL and Caruso. http://www.styleforum.net/t/94886/ralph-lauren-lines-and-who-makes-what-a-definitive-guide