The off white beige was used for different things but still it's considered feed sacks. Planting grains such as wheat and hay seed were often sold in the white sacks, some of the smaller ones had sugar and flour in them. Animal feed was usually in the printed sacks. They are all still grouped in what's refered to as "feed sacks". The fabric that you posted looks to be authentic. The weave looks right, the thicker thread used looks right. Sometimes the holes along the edges have been cut off so you can't always use that to go by. If they're there it's easy to tell. If not then you have to look at the weave, the threads and the "feel" of the fabric. Even then depending on the company, the feed, etc., the fabric varied. Some of it was coarser than others but none of it was the quality that you find in the reproductions of today. Looks to me like you've hit upon a jackpot, congratulations. BTW, these fabrics wear like iron.