Threadless stem model. 1.125" steerer clamp, 31.8 mm handlebar clamp. I think it's threaded for 5 mm bolts on the clamp, & 4 mm bolts on the faceplate.

Absolutely unacceptable for 3D printing. This will certainly break. A ton of torque is being transmitted through this thing, & a significant fraction of your body weight. Use this only as a model. If you're machining this, that's okay, just check the wall thicknesses to verify it's safe. I would still be cautious. You could die on this thing if you're not careful.