Chevy casting dates break down as thus...
A-L=Month
next 1 or 2 numbers=date
next 1 or 2 numbers=year
Most times they were seperated by spaces, but sometimes not.
Your heads are dated F256 and C216, this would be
F-June
25-25th day
6-1966
and
C-March
21-21st day
6-1966
In the early '70s Chevy began using two numbers for the year on some components, so it was easy to determine what the year was.
For example, my block is dated B 3 0, but the date on my intake is 1-14-70 (aluminum intake cast in the winters foundry, not sure why they used a "1" instead of an "A")