Marking the envelope as "N 400"

Did you guys mark the envelope sent to USCIS with "N 400" on top of it?

Any cover letter needed?

Thanks

If you send it to the street address, the address would be

USCIS
attn: N-400

If you send it to the P.O.Box address, I believe that P.O box is soly for
N-400 packages.
 
I did include a cover letter and did write "N400-Application for Naturalization" on the top of my envelope. Neither is needed though.
 
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