A farmer was asked at his road-side stand how he could sell rabbit-meat sandwiches for only $1 each, and the farmer admitted that he used a bit of horse-meat to "extend" the rabbit-meat. When pressed, he admitted that the horse-meat to rabbit-meat ratio was about 50:50. When he was asked how he measured that ratio, the answer was "one horse, one rabbit".
Wow... I thought the information would be easy to find. It's not.
The USDA website was not particularly helpful, but I did locate a reference to '9 CFR 319.15(a)' which covers allowable fat content in ground meat. I could not find the actual document, nor could I find any documentation on how fat content is to be determined, nor the allowable variability.