This is a very compact "key hole hanger" which is an easy way to mount frames, mirrors, cabinets etc.
There are two types provided: a single hanger which is 15mm wide x 25mm long x 6mm depth, and a dual hanger which is 15mm wide x 50mm long x 6mm depth. The dual hanger is designed for two screws mounted 25mm apart.
To fit them, use a 15mm Forstner bit to drill two holes to a depth of 6mm. For the single hanger, two 15mm holes need to be drilled 10mm apart (to give the 25mm overall width). Then simply use a chisel to remove the small amount of waste material remaining after drilling (they are very easy to fit, because there isn't much material left after drilling), and glue them in place. They should be a fairly snug fit straight off the printer, and you can tap them into place with a wooden mallet until they are flush with the surface.
The key hole will suit a screw head up to a maximum of 9mm diameter, and the key slot is 4mm wide. The key slot includes a shallow ramp which is designed to pull the fixture flush against the wall as the screw enters the slot.
Although I've not tested these to destruction, they are designed to be fairly robust. They are designed so that the screw head comes to rest against the solid outer wall, which in turn should be flush with the surrounding wood. They are best printed with 100% infill of course.