This is the SL-15 V4.8.A by Hoffman Tactical.

The SL-15 is a 3D printable AR-15 lower receiver capable of being used in the same capacity as a factory made aluminum receiver. A low cost FDM 3D printer, basic assembly and computer skills, and the factory lower parts are all that is required to complete the lower. A factory AR-15 upper is required to complete the rifle. The goal of this development was not to provide a complete firearm, but instead to provide the regulated receiver in areas where the other parts are easy to obtain. For those who may have printed earlier lowers, V4.8 has several major design changes. The normal overhang design rules were departed from in the grip and trigger guard area for the sake of better ergonomics, this results in additional precautions while printing the lower that will be covered in more detail in the printing section. It should be noted that in areas where ergonomics were not a concern the overhang design has been further improved, such as the buffer threads. Built in supports have not been included in V4.8. In there place support enforcer files have been included to assist with the generation of slicer generated supports. The decision to not include built in supports was made due to the excellent organic and tree supports now available in slicers. With the aid of the support enforcers and the settings in this guide, the process of generating support material is almost as easy as using the built in supports. Rather than separating the developer files into another release, they have been included in this download along with the more commonly used vector files. Both STEP and SolidWorks Part files have been included.