The Lego UCS AT-AT has one part missing and one part too many. Could that be?

Hello, I've built the set and noticed that there's one extra part (which isn't actually a spare part), and one part is missing. A round 6×6 gray baseplate is missing from the seven bags. There's also one leftover part. It's not a regular spare part, but a connector with 5 holes and 6 on the other side. It's 1×1 wide and 11 pieces long.

I disassembled the legs (which are the parts) five times and checked them against the instructions, and I was getting frustrated. Of course, I also checked the floor, etc.

Could this be a manufacturing defect?

Thank you for your help