The following site: http://www.tocmp.com/ has sales brochures of cars going back to 1904. Having the right cars, or billboards for them, can help date your RR. Some interesting ones: 1949 Dodge Power Wagon, 1961 El Camino, 1904 Packard, and others. They are presently updating the site, and have 25,000 + brochures to download - free!
Thats a great website loaded with info. I’m going to copy the link and post it on some car forums I’m on.