If you go back and look at one of my links, there were several PII Springfields, the Henley being one of them. Chassis (without wheels and tires) & mechanicals built in Derby then shipped to Springfield for completion and bespoke fittings.
http://www.rmauctions.com/mo12/monterey/lots/1931-rolls-royce-springfield-phantom-ii-henley-roadster-by-brewster--co/1052448
"On October 18, 1919, Rolls-Royce of America was launched in Springfield, Massachusetts and produced 2,944 Silver Ghost and New Phantom cars between 1921 and 1931. When production of the Phantom II ended in England, Springfield was left with a sparse inventory of Phantom I’s and a few imported Phantom II’s, so the Derby factory completed a series of “Americanized” Phantom II chassis and sent them to Springfield. "
Here's an excellent build of the Monogram kit.
http://www.modelcarsmag.com/forums/index.php?showtopic=14314