Very rarely a very very small batch of Gen 3+ engines came with "thick block", and all yankee 98+ 5S-FE's come with thick block.
The batch of gen 3 "thick walled" are very random, there seems to be no pattern on why they pop up. Luck of the draw.
This is where they usually break
http://i213.photobucket.com/albums/cc16 ... C00219.jpg
Also the 3S-GTE block are believed to have a very thin wall between cylinder 2 and 3, and the material inside is actually in a hollow-thru pattern, not much "meat" supporting between the 2 cylinders, causing it to do bad things when high cylinder pressure is present.