This probably won't help much since I don't have all my RRA stuff yet, but from talking to a lot of people, the new RRA block supposedly looks exactly like the one Colt is now using.

I'm building a 9mm using a TROS barrel, so I wanted to buy all the other parts and assemble myself.  Don't even think about ordering directly from RRA.  They are out of damn near everything.  I called Eagle Firearms for the parts I needed.  The RRA flattop uppers are out of stock and RRA can't tell you when the heck they will be in stock.  Eagle has an upper from another company that is in stock and is better, in their opinion, than the RRA.  Just call and talk to Chris about it.  I totally trust Chris and Kendra at Eagle Firearms.  Awesome folks, great products and service.

Regarding the RRA mag block, Eagle has those in stock too.  RRA does not.  Chris, and a few other people, told me the block looks identical to the Colt one.  It inserts from the bottom and uses a set screw to keep it in place.  I'm told that is should be very secure in the lower.  Press the mag release button on your lower and the mag drops free.  

Eagle has converted (by RRA) Uzi mags and Colt mags.  The Uzi mags are converted by cutting a notch in the mag for the mag catch/release.  I'm told they should work fine.  If they do, I'm sure it would be easy to use one of these mags as a template for converting other Uzi mags.

I hope this helps some.  Keep in mind I do NOT yet have any of this stuff in my hands.  I'll be sure to update this post when I do.  I've already got the lower built, I'm just waiting on the upper receiver, barrel, mag block and mags.

If I've posted anything incorrectly, someone PLEASE make me aware of it.

Yes, I am told that is the case.  I will confirm when I get all my stuff and start putting it together.

I should probably document everything I do along the way to help out other guys wanting to do a 9mm AR.  There is a lot of information out there, but it can still be confusing.