Do a search with the BYC search engine at the top right corner of the screen and you will find atleast 3 threads that has already been started about them. They are from Mt Healthy hatchery and are only being sold to Tractor Supply to sale in their stores. Here is a quote from an email when I emailed Mt Healthy about them We are carting the tetra tints just for Tractor Supply, we do not have them listed on the website, they are a high producers of creamy brown eggs, they are white in color maybe with some darker spots on them and the adult hens will reach app 5 lbs Thank you, Mt Healthy Hatcheries So they would bacically be a cross of a White Leghorn with another production breed, similar to the Austra Whites that Cackle is carrying this year that are a cross of a Black Australorp roo on White Leghorn hens.
They are probably someone's specific strain of sex-links. There are many; Bovans Brown, Hisex Brown, Tetra-Americana, Shaver Brown, Dekalb Brown, Babcock Brown, etc.