Ok you got me wondering so I've been doing a little research what I've found is these numbers are usually under the timing cover but I have found some occurences of them being cast into the side as well.
When under the T-cover you will usually find 010 AND 020 this indicates metalurgy 010 = an extra 1% tin added to the metal in the block
020 = an extra 2% nickel added which makes these blocks stronger
The 509 is for the 2 bolt main 400 which is actually better than a 4 bolt.
It seems the outer 2 main bearing bolts on the 4 bolts cause weakness in the block.
The main thing I have found is this is what you want to see for your casting #