340 X head date code

My 1968 340 X heads have dates codes of 09047 and 09187. I know the "7" is 1967. But I thought the preceding three numbers were supposed to be which day of the year (out of 365) they were cast. Obviously, that can't be. Is it the month and year? So Sept. 4 and Sept. 18?

9/4/67 and 9/18/67 normal...