Bicycle Seconds are the USPCC factory seconds. These cards may have minor
printing defects; for example they may be printed slightly off center. Early examples
had a specially printed ace of clubs with "Seconds" printed in a
circle around the club (see the example below). Some later issues had "Seconds" rubber
stamped on the ace of clubs, but most seem to have no indication other than
the special box.
The box pictured below with the hashed two in the background is a rare U.S.
Printing Company (US8aa) deck. It is not identified as a "Seconds"
deck, but that seems likely [photo courtesy of Steve Bowling].