08:17 Jan 31 

"fold-back" could be a linguistic misrepresentation of "feedback". Although there is insufficient context to be sure, I am now fairly convinced that that is probably what it is (as per the drawings in my link below). It puzzled me that the face connections would be taken out to one side only since many such discs are bonded to brass plates which can (and do) accept soldered joints. I have used all manner of piezoceramic discs in the past.

01:11 Jan 31 

Thank you for your comments everyone. In fact I wanted to know how to say it in Japanese but I posted here because in French-Japanese section I rarely can get a suitable idea. Now I found an answer in Japanese. And I happened to encounter the word in English, "Fold‐Back Electrode".

09:06 Jan 30 

Some piezo-ceramic discs, in addition to the silvering on both faces, have a small "finger" which generates just enough voltage to provide a positive feedback signal, generally for ensuring that oscillation occurs precisely at the resonant frequency (which, in the case of hydrophones, changes when dunked into water). However, in this particular case (a diagram is always helpful) and accurately described by Tony, it seems that the purpose of the "retour" is simply to enable electrical connection on one face only. But see http://www.ryston.cz/pdf/avx/piezo.pdf (p. 10) for an illustration of a feedback tongue.