This is a CAA Upper known as the Micro, model MCKSIG.
The next generation of Micro Roni is here! CAA improved past generation models into the ultimate conversion kit. It's Micro Roni Generation 4" and they call it Micro Conversion Kit" or simply MCK. This Micro Conversion Kit fits multiple handgun models" into one platform" and helps convert your pistol into the ultimate Sig Sauer gun. It is significantly improved from previous generations of the Micro Roni Stabilizer.
Features:
No pistol disassembly required – Just place pistol into the kit, lock, and go
Allows for multiple handgun models into one platform
Ergonomic Finger Groove Grip for more stability
Top Picatinny rail, allowing a combination of sights and/or optics
Side Picatinny rails for mounting additional accessories, i.e… thumb-rests
Ambidextrous, fast and easy handling