340 Casting Number Questions

I've got a couple of 340 blocks that I just acquired. One of them is cast date in 69. I can't remember the actual date. The casting number is 2780930 and 340-3. The other one has a casting date of 7-7-70 and 340-5. Does anyone know what the -3 and -5 stand for? Back in my auto parts days if I remember on a 318 the automotive casting was a -1 and the motor home and truck block was a -2 or a -3. It seems to me the 360 was the same. Things are a bit fuzzy after this many years.
the other questions deal with heads. I have two sets, one of which is supposed to be the original off the Duster. They are 915 J heads. Some references say the 915s were 72-up 340 and 360 1.88s while the 71 340s had a different casting. Two of the heads that came with the blocks I got are cast J but backwards and one is an X and one is a O. I didn't get the casting number off any of these, just the letter. What is the difference between the J, X, and O? Any help is appreciated. Thanks.

Emil