Jeff, I just noticed something in the photos. Are my eyes deceiving me, or do I see two spots where there's what looks to be a stub of a sawn off 4x4 post tucked up between the header joist and some blocking? Look at photos 10 (left end of that wall) and 16 (right end). It looks like there are two sawn off 4x4's up in there, between the header joist and blocking.
PLEASE check, and post back.
I'll bet you lunch that the original framer installed 4x4 posts to make up for a missing header, got dinked at the inspection, and was forced to install the wall. But tell me I'm wrong. My eyes could be deceiving me.
You are seeing a 4x4. Its the railing post from upstairs.