After looking for hours I can't find any cast number on my 2001 cummins. I have looked on both driver and passenger side and have found no 1in tall numbers. I have however found a 23 on the drivers side a few inches above were they say to look for a 53. Also above the 23 are the letters G I. On the passenger side I have a 1in tall "A" and above the "A" a circle made of dots. Any idea what block I could have?

 

Following. I went to buy a 2001 and searched for the 53 and I had exactly what you found 23.