Right, the 1956 Lincoln Continental MK2. Model Power makes that and a few other 50's vehicles as well. A year or so ago they had a run of 55 Chevy Bel Air 2 dr sedans in a variety of 2 tone paint jobs and a 53 Caddy convertible. They're still around here and there and the Caddy's are often listed in Walther's Flyer. Minimetals has a 59 Ford 4 door sedan in a few 2 tone colors as well as a police and taxi version too. If your RR is just barely into the 60's Model Power has a decent 62 or 63 T-Bird.
Woodland Scenics has a couple of late 50's Chrysler Corp cars as part of their Scenic Accents. Somewhat pricey but you usually get a few people and other small details.
Alloy Forms made or still makes a few 50's cast metal car kits if you're into painting and assembling them a well as making up some window glass.
The 50's stuff is out there but not in the abundance of more modern era vehicles so it takes a little hunting down sometimes.
Check on Walther's web site under manufacturers for Model Power, Minimetals or Classic Minimetals or do a broader search for HO vehicles.
http://www.walthers.com/