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Have a used ProMag 9mm adapter block. Decided to go with a dedicated 9mm lower. So, don't need this anymore. It drops in from the top of the receiver. You have to remove the bolt catch to install. Takes Colt SMG magazines.
Had feed problems with my cmmg upper. But I think the issue is more related to the magazines and not the adapter block. The mags, somehow hold 11 rds (came that way). After the 2nd to last round is fired, the last round pops out of the mag, doesn't feed, and the bolt catch activates.
The adapter block is mounted in the upper at the moment. So, no pictures. This one is from google images.