What do you want to know? Should be a 16" pencil barrel with slickside A1 upper and CAR stock, identical to an SP1 carbine except for the caliber. Pivot pin may be a large two piece screw like on the SP1. Mag block should be a two pieced that's pinned in place. Mags are 5/20 or 5/32 for Colt marked ones and used to command a very heavy premium. They still may since supply doesn't really seem to be any better. 10" uppers were available on the SMG variants (RO634,635 and 639) and they do surface from time to time from police trade ins etc..
The only thing to really be aware of is that the Bolt Hold Open device is a special one with long tail. They are prone to failure because of overtravel of the buffer. You can replace them with regular ones but then the bolt won't stay to the rear on the last round. Not a big deal in my opinion. I have a mixmaster 9mm built on an SP1 upper and lower and use an aftermarket magblock that doesn't use the BHO on the last round. I may modify it to work in the future but it doesn't ruin the fun for me.
The most reliable 9mm AR there is.