I have a CM code A2 upper with a windage knob that has no knurling, but instead, two slots that look like they are intended for some kind of spanner/tool to rotate?

Is there an A2 rear sight tool?  

I have never seen a specific tool to move that.  I would look around a pick up a knurled one if it is a big deal to you.  I guarantee you, that a tool that size would have been quickly lost.  If that is factory, not one of Colt's better ideas.  What model gun is that, it does not look like a factory rear sight, it looks like some of the after market stuff I have seen.

I have seen that sight before, but never on a Colt.

Does not look like it would be hard to move with a FMJ bullet, like we used to use on some of the older guns.

Derp.  Apparently I've asked this exact question before, last year:

https://www.ar15.com/forums/ar-15/A2_Rear_Sight_Windage_Knob_Variant_/127-695334/

CrackedCornish/Jaygee, if you see this, how do you turn yours?