This lot is a  Platte River 400 Grit Diamond Wheel 4 inch with 1/2" center hole and 1/2 " total depth. It's hard to tell but I guess its closer to 1/2" than 3/8". This came out of a woodworking shop I really don't know much about it. New Condition with a  4" by 3/4 inch square whet stone.I'm guessing it is quite expensive. Happy to try to answer any questions.  Thanks for looking and God bless the USA.