The bolt running through the center appears (repeat: appears) to bolt the ends of this transducer together. Which defeats the goal of a transducer of this type (to vibrate in the axial direction).
How does this transducer couple the brown ceramic disc piezos to the "business end" (upper, in the main photo)?
Someone suggested that the bolt is actually a threaded tube with some kind of driver stud in the center (if you look closely there does appear to be 2 separate pieces that make up what looks like the "bolt").
??