When general sales taxes were introduced in the US from 1930 onwards (starting with Kentucky and Mississippi) there was no easy way for a retailer to levy the tax on small purchases. For example, you might buy something with a retail price of 10 cents on which the sales tax would be a fraction of a cent and had no way to pay it. The retailer would nevertheless still be liable to pay the tax to the government based on the total of his sales. If he sold a lot of small items, he effectively lost money on each of those small sales and it could soon mount up.
These tokens were the solution. They came in various values and yours is valued at “1 mill” (a thousandth of a dollar, or a tenth of a cent). So if, for example, you made a 10 cent purchase liable for sales tax in fractions of a cent, the retailer would ‘overcharge’ you at 11 cents and then refund the ‘overpayment’ in fractional cents with these tokens. He then wasn’t losing out on what he had to pay the government.
Not every state had (or needed) these tokens. About a dozen states issued them. I think for Oklahoma they were issued from 1936-1941 and were formally ‘demonetarised’ in 1961. In most states they were discontinued during WWII as a result of the additional complications of ration tokens and stamps.