Welcome Plum (didn't want to call you Crazy

):
I'm not sure about the "special" rate? The standard rate in 1945 was 1-cent for a postcard, and 3-cents for a one-ounce letter. I don't know about the fee for additional ounces.
A better approach might be the stamp itself. Can you post a scanned image? Can you describe the stamp?
During that era, I didn't think there were any U.S. stamps with fractional values.
KirkS