probably the best blocks to get have the 010 or 020 under the timing cover. What these numbers mean is that they have 1% tin and 2% nickel.The tin is used to help the molten iron pour and flow better into the mold. These numbered blocks are the least prone to cracking. Because they pour more easily, they also have less pro0blems caused by hot spots that is caused by porous metal. The nickel that is in the blocks work hardens as the engine heats up and cools down this makes them more resistant to bore wear.
If your block only has the one set of numbersthat are either a 010 or 020 this will mean that there is no additionally added tin but it does have the 1 or 2% additional nickel. If there is no number on the block of coarse that means there is no additional tin or nickel, so these blocks can be expected to wear out faster.
So this should help you work out what you have got.