it's the control number "2001..." see:

来源: 于 2006-07-17 07:25:20 [旧帖] [给我悄悄话] 本文已被阅读：次

"Part 3, Section A — The “visa number” is the control number written in black on the visa stamp."

not the red one. the control number contains USCIS needed information whereas the other does not.