Potato tax US revenue stamps were required by the Potato Act of 1935, an amendment to the Agricultural Adjustment Act that became effective December 1, 1935.
Farmers that exceeded their federally established quotas were required to pay potato tax on the excess volume with the tax paid stamps. The tax amounted to 3/4 C. per pound of potatoes.
These stamps were available and valid for such a short period of time (about two months) that they were never actually put into use. However, many stamp collectors and dealers bought mint condition sets of them, and even today, these stamps are quite affordable.