What is the kashrut authority behind the K-כשר symbol that is now used on Kirkland Signature milk sold at Costco?:
In response to my query to Costco they sent me the following information. K-כשר is Rabbi Aryeh Spero. He runs Quality Kosher Supervision and is located in Canton, OH. The milk comes from Superior Dairy Inc in Canton, OH.
Rabbi Spero also has another symbol - The Kaf-Q. I will try to see if I can add in the letter from Costco.