The "USCIS #" on your current green card IS the Alien Number. That is the number that you should put in the I-751 form.
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The number on the CG is the A#.
NYC EXPERIENCED IMMIGRATION ATTORNEYS www.myattorneyusa.com; email: firstname.lastname@example.org; Phone: (866) 456-8654; Fax: 212-964-0440; Cell: 212-202-0325. The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter.
The number appears on the resident card. I also appears on some of the filing receits used for the petitioning process.