I don't know , but these days super glue gel seems to be common .
When we were installing these transducers, back in the day, we used Elmer's School glue (water soluble in case they ever needed to be removed). Hope this is of some help. Stuart
You might also try the 3M foam adhesive (double sided) tape. Some of the pick up makers used this as well. It can be quite tenacious.