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With an IR3 visa, it would be the office that issued the COC. Buffalo.

With an IR4 it would be your local office that issued the COC after readoption and filing the N600.

That said, considering the length of time it has been, I would bet that most likely it has been sent to the National Records Center in Lee's Summit MO for filing.

You can email Child-Citizenship-Act @dhs.gov or call 716-551-4741 ext 6130 (this is the Buffalo office) and if you give them your child's A# then can tell you exactly where the packet is so you can be sure. Just be sure to include your child's A# as that is what the packet is tracked by. It is on the COC (not the COC number, but the A#) or on the visa in his passport.