An old-style magwell but allows for magazine to be topped-off with darts when the breech is open. This is accomplished by moving the inserted position of the magazine DOWN by 1/8 of an inch. So I'm not currently sure if this will be compatible with every aftermarket mag or mag adapter that exists. I really only intended to use this with straight-18 mags so as to avoid having the carry any spare mags.
This requires all of the included parts and is NOT COMPATIBLE with the takedown function. So you will need to review the older revisions of the instructions for the first few pages of the assembly.