After finding a g-g uncle (maybe g-g granddad) who worked the phosphate mines in Maury County TN , early 1900s (now home of the Spring Hill Saturn plant ), I found possible old company names: Hoover & Mason Phosphate Co., Victor Phosphate Plant and Virginia & Carolina Phosphate, all in Mount Pleasant, TN, and Tennessee Valley Phosphate Works (this one probably from the 1940s) in Columbia TN. Looks like some of these became Monsanto Co. so maybe they have a way-back file. I'm looking, will let you know.