A "mill" is 1/1000 of a dollar... 1/10 of a cent. If you had 1.4 cents due in sales tax, you paid the two pennies and got one of the green and one of the red plastic mills back in change.

These plastic tokens were our home poker chips in the 1940's. Before the war, the Missouri mills were made of zinc.

DonL
Univ of Missouri, 1959