What famous 1930's crime was solved by tracing serial numbers? Bonus: an item of numismatic ephemera which led to the arrest of the suspect was sold in what recent auction?

Tom Delorey, Bill Swoger, and Bob Cochran all correctly answered the question, although no one collected the bonus. Bob Cochran summed it up best:

Here's an interesting web site about the crime and trial:

The bonus answer? Lot 1660 of the June 19, 1999 R. M. Smythe sale at the Memphis International Paper Money show was a copy of "The Official List of U.S. $5-$10-$20 Notes Paid by Colonel Lindbergh to the Kidnappers of His Son" The 57-page pamphlet lists the "serial numbers of all the small size notes comprising the Lindbergh ransom money." The lot was estimated at $250-$300, but brought $1660.