This was GM design chief Bill Mitchell's car and, using the original 1963 Riviera as a base, had a lowered roofline and lengthened hood. (Silver Arrows II and III were relatively minor modifications of production Rivieras.) It is owned by Sloan Museum.

source: www.buickclub.org.au

The 1963 Buick Riviera Silver Arrow I Buick concept car was a production Riviera coupe specially customized by GM styling head William L. Mitchell. The roof was lowered and the front fenders lengthened several inches housing the concealed headlights. The car served as a personal vehicle for Mitchell when not being shown, much the way early Buick concept cars had for his predecessor Harley Earl.

There were actually two more versions of the Silver Arrow I. The Silver Arrow II and Silver Arrow III incorporated unspecified, but relatively minor, changes to the production Riviera.

source: http://history.gmheritagecenter.com