the -1 is probably the core #. Since they are sand cast molds, there are more than 1 mold, or, there are more than 1 cavity per mold.

I also think the 666 casting would be correct. 301'sre 68/69.

A friend has a 5k mile original Bee, I can look at his intake today. He's just few blocks away. Don't know the build date, but I know its a St. Louis plant car.