Can't remember where I found it at but it was probably at the DMV office. I was in there a lot when I was getting my kit car sorted out and legal. It didn't have an interior rear view mirror at the time and I found out as long as I had one mirror it was legal. The law doesn't specify it's location either so it can be anywhere.
There are lots of weird laws here that nobody pays attention to like doors are required to open out sideways and not up, they can't go above the roof while opening even if it is a factory design. Fog/driving lights that are not amber are illegal. And a few others that would fail most factory cars for a true legal inspection.
I am in Logan county so I am not that far away from you.
Missouri law says I don't need ANY mirrors, as long as I can see out the back or the side of my vehicle. So, as long as I can lean out the window, and see behind me, I'll be fine! Must have been written in 1900. The only other place I'm likely to drive is Kansas, which requires only one mirror. Hmmm, maybe I *should* wipe that driver's mirror.