Slant Six block casting numbers???

I have three complete engines..
Block one 2806830-9 BH 66 dart?
Block two 2806830-8 BH 64 valiant
Block three 2806830-4 BF 72 scamp

I know the casting 2806830 are all 225 slant six

What does the last number mean and the letters mean?

I looked it up and the best thing I could get where they are all 68-75 blocks. BH blocks are better to build becuase of thicker casting?

I know the one from the scamp is original, could the others from the 66 and 64 been replaced at one time? Are the other two original to the 66 and 64? Where can I located the year of the blocks? Are these 68 and up blocks?