What do the casting numbers 64-5-M 9 11 12 20 41 83 60 refer to on the bottom of an old Kohler clawfoot bathtub?

I think the part of the number codes '11 12 20' refers to the casting date. I have restored two Kohler tubs with the last number in the sequence being '14' and '15' from residences in Richland Center, Wisconsin that I know were built/upgraded around the 1917 - 1920 time frame. But, I have found a definite reference for this yet.