Yes they are date coded as are a fair amount of other things as well.
I have pictures here

http://s872.photobucket.com/albums/a...ing/?start=all

showing a few old Holden parts with dating on them.

Heads and blocks would I expect have been cast at different times if not days
If the heads and block are from a close time frame to each other there is more chance of them "matched" as such.
Cheers Paul.