The Answer:

According to the U.S. Treasury Department:

"The engraved die of the Treasury Building vignette was completed in the early part of December 1927. The engraver was Louis S. Schofield. There are four cars included in this vignette. These cars are of no specific make or model and each one is a creation of the designer who prepared the original model which was later used by Mr. Schofield when he made the original hand-engraved die of this vignette."

This question was originally posted in 1999. The $10 bill has since been redesigned, and no longer has any automobiles included on the back. Here is a link to more fun facts about this country's paper currency.

—The Fact Monster