Just finished the I-751 + N-400 combo interview at the St. Louis, MO office.
Went in at 9:10 and came out at 9:31. US citizen spouse was present for the first 5 ish minutes and then it was just me later. Officer just verified all the dates and asked 6 questions and then verified my driver's license + passport + (expired) green card + SSN card. They took away my OPT EAD (was on F-1 visa before I met my spouse) and the EAD/AP combo card, lol.
They did not take any copies from me that I had printed, and didn't look at any other original documentation.
At the end they gave me a sheet of paper that said they have recommended my approval and that they will send oath ceremony appointment via USPS. Case status changed for I-751 as approved and for N-400 as oath ceremony will be mailed.