Looking at the two uppers I noticed a couple of differences,

1: The 6400 has A2 on the carry handle, a "C" and keyhole like thing on the upper.
    The 6920 doesn' have the A2 and has what looks like a "D & an A" on the upper.

Does that mean the the 6400 is a Cerro and the 6920 Dimaco?

2:  The 6400 is marked .223 on the lower and the 6920 is marked 5.56.
     Both lowers say Colt Defense.

   
    What are the differences?    Thanks!!!

I think you're right about the forgings. Diemaco and Cerro both make forgings for Colt. Cerro makes them for Bushmaster and Rock River arms as well.

The 6400 uses the civilian designation of .223 for caliber while the 6920 uses the NATO designation 5.56, both of which for all practical purposes are identical. The 6920 is made for the civilian market and the 6920 is for the LE/Military market.

Colt split up into two companies two years ago. One serves the commercial market (Colt Manufacturing) and the other for the LE/Military market  (Colt Defense). Colt Defense is the company that makes the black rifles. Colt manufacturing makes the handguns like 1911 and wheelguns (SAA, Python, Anaconda)

So how do the MT6400 and 6920 compare in terms of quality/fit/finish? I bought a MT6400 only  to get rid of it and get a couple LE 6920 HB's. The one sample of the 6400 I had seemed shoddy but probably because I got it from a shoddy dealer. I think he slapped it together from parts to make a "ban free" rifle.  The LE 6920 HB's are absolutely flawless, a great example of fine machinery. People have blasted Colts for quality/poor color match etc. But these two samples are as perfect as can be.  At least as good as the other top tier makers if not better.