[1971WARBIRD]

yes, just one head, they just added the fitting for the heater on the passenger side once they decided which head would go there.
yes the dates changed every day, which caused some interesting things. I have a couple 1968 number 62 heads that the casting numbers and dates are up side down.
only difference I know of, is the earlier heads the intake ports were different. instead of being flush with the side where the intake bolted on, they were cast with a depression so the air could pass between ports. as I recall, that ended about the time the A series (January) heads were cast. it has been talked about on here, but I can't remember what they decided it was suppose to do.