Sales Tax Token.

From the website:

Sales tax tokens were made in great quantities starting in 1935 in order to give change for sales taxes. Sales tax resulted in the final price of items having fractions of a cent. For example, purchase of a $1.25 item, taxed at 3%, would cost $1.2875, or $1.28 and 3/4c. What to do? Rounding up to $1.29 would result in a "unfair" profit to the seller of 1/4c, but rounding down would be unfair to the seller by reducing the profit by 3/4c. The solution was to provide tokens denominated in fractions of a cent, or "mills" (1 mill = 1/1000 of a dollar, or 1/10 of a cent). So in the above example, the customer would pay $1.29 and receive 2.5 mills in tax tokens as change. If the next purchase came to $3.4325, the customer could pay $3.43 plus the 2.5 mills in tax tokens. As you can imagine, people did not like having to carry a second set of coins, and to further complicate matters, different states issued different tax tokens. The use of tax tokens declined and was finally discontinued in 1961, and people basically decided not to worry about fractions of a cent.

Link | A huge collection of images at TaxToken.org - ThanksTom Van Vleck!