Where to find Alien registration number on I-797 C?


I am filing B2 visitor visa for my parents and need my alien registration number.

My H1 B was approved in Nov 2011 and it is not stamped, I just have a copy of approval notice I-797 C form.

I-797 C form does not contain Alien registration number along with my H1 validity, where should I find it?

It starts with A. Below is where U can find