The "53" comes from a casting number on the side of the block and is used to identify the problem blocks. Supposedly it affects about 100,000 blocks, but not only in Dodge Rams.
1. Brazilian Blocks are cast by a company named "TUPY" and are marked such. They are identified with cast evolution numbers. #53's are TUPY's. #54 and later evolutions were improved to correct the block issues that the 53's had.
2. Mexican Blocks were made by "Teksid" these blocks to my knowledge never had a problem with cracking, as they are heavier duty by trait, but they did follow the evolutions and got thicker. These blocks do not have the 2 digit "cast number" embossed on the side, instead a series of numbers that are unidentifiable to me. The Mexican blocks were rumored to even be harder to machine do to a harder cast overall, basically better quality in my opinion.
Brazilian Tupys always seem to outnumber Mexican blocks during the ISB's production probably about 8 to 1. So basically if there is a pattern number its Brazilian, if there is a series of small numbers its Mexican.
By the way the Mexican equivalent of a 56 pattern was the Heaviest ISB block of all. This means if you have a 2002 without a #56 on it and instead it has a series of numbers that are smaller and hard to identify, you got the the big boy!!!