that's my question

USCIS or DOS case number?

USCIS is Three letters followed by 10 numbers. The leters indicate which service center is working the case, this is on your NOA1 letters.

DOS is also 3 letters followed by 10 numbers, the letters indicate consulate where the visa will be issued, in your case (GUZ) the 10 numbers have to do with when the case was assigned. SEE: http://travel.state.gov/visa/frvi/glossary....html#cancelled

You can get DOS case number a few weeks after NOA2.

Lastly you may be interested in "A Candle for Love" that site specializes in Chinese immigrations issues, and particularly the consulate in Guangzhou. The FAQ is second to NONE.

http://candleforlove.com