Question about Lionel Mint Cars with "Paper" Loads?

The majority of the Lionel mint cars have had a cargo of “gold” or “silver” bullion.

Lionel has also produced a few cars that have a “paper” cargo, such as the:

19696 U.S. Treasury Department U.S. Savings Bonds in 2003

19697 U.S. Bureau of Engraving and Printing $1000 Notes in 2004

29607 Las Vegas Mobile Vault $1000 Notes in 2005

29634 Federal Reserve Mint Car

UNION PACIFIC U.S. Bureau of Engraving and Printing, 2007

Lehigh Valley Mint Car, 2008

Does anyone know what type of materials is used in the load in these types of cars with “bonds” and “notes”?

Are they stacks of paper printed to look like miniature currency? Is it a plastic load that is printed and/or decaled?

Any info is appreciated.

Note: the info on these cars was obtained from the TCA e-Train article: “Mintz’s Mint in the Box Mint Cars” by Bob Mintz