the D5 cast code basically says that it's a 1975 part. there's no way that head came stock on anything before late 1974, but the casting could continue well on after 1975. there is also a casting date cast into the head, gman26 says he found a "71" to which Brian S replied that it's prolly a "7I" meaning Sept of 77, but i don't think "I" is used in the date codes, it would be skipped to avoid confusion between the letter "I" and the number "1". a date code for Sept 1977 would look like 7J15, 7 is the year of the decade, J is the 10th letter for the 9th month because I is not used, and 15 would be the day of the month.

the 71 that gman26 found is prolly just a random number, there are a lot of them on Ford castings. but as Dennis Miller says,"i could be wrong"