These are pickups for 36 or 37 string autoharps, for installing with internal wiring.
Requires drilling and soldering skills. The jack is normally installed through the treble edge of the autoharp, drilling a 15/32 inch hole. Hole is also drilled through the top and the wire is fished through to be soldered to jack. We use a small amount of epoxy or gel CA glue to hold jack in place. On some autoharps a space for the end tabs on this needs to be made in the bar holders.
See a video on how we install these here: 
Install A Pickup