If the ultrasonic horn/sonotrode you have to run was made for some other piece of equipment you will possibly need one of these. Most 15 kHz and 20kHz ultrasonic equipment made in North America and much of the rest of the world uses 1/2-20 to join tooling components. Both 1/2-13 and M20-2.5 are rare. Tuning specifications vary somewhat from manufacturer to manufacturer, but in most cases horns/sonotrodes from one manufacturer will run on equipment of the same frequency from another manufacturer without re-tuning, but there are some exceptions so approach with caution.
This specially hardened stainless steel step stud has a screwdriver slot on both ends and may be tightened into either component before assembly. No warranty.