I'm looking to change the spark plugs in my 70 elky(generic remanufactured 350)

For the Bosch plugs I'm looking at, there are different numbers for 1970,71, and then they have the same number for 72-77.

Anybody know what the differences in these plugs are?

I have a mallory unilight style(non-points)distributor if that makes any difference...

Thanks-
Ben