The car could be the production coupe based on the LF-CC Concept, shown first at Paris this year. Given the nomenclature, the RC350 could bear a 3.5-liter V-6 like other Lexus -350 models, as hints Lexus Enthusiast, or, should the brand choose to follow its German counterparts further into the realm of more creative numbering, the 2.5-liter four-cylinder and hybrid drivetrain found in the LF-CC.
On the other hand, however, the LF-CC Concept could simply come to us as an IS Coupe, in which case, the RC350--again, using the name as the cue--could intimate another model altogether. The only similarly-named vehicle in Lexus' lineup is the RX350, and while the X is used to denote Lexus' SUV-like (or crossover) vehicles, the first letter generally denotes the series: CT, IS, GS, LS, RX, GX, LX etc. Assuming Lexus would want to avoid clouding the RX brand with a non-crossover vehicle, the RC350 could end up being a compact, coupe-like crossover such as those we're beginning to see from the Germans to complement the RX.
Both of the above cases are, however, little more than idle speculation at this point. If you were to apply Occam's razor to the matter, you might just come to the conclusion that RC350 is nothing more than a typo on a patent renewal--the X key is right next to the C, after all.