I was looking here
Transducer Shield and Saver - See More Fish, Catch More Fish! - TM-ONIX-1
and read this
IMPORTANT NOTICE: Humminbird has a redesigned Onix transducer which will NOTfit into TM-ONIX-1. The redesigned transducer number has an (RD) at the end of the transducer number "XT 14 20 HDSI T (RD)" . The redesigned transducer has a lip all the way around the top and bottom of the transducer and has a protruding epoxy trough at the back. The photo below shows the redesigned transducer - please be sure to look at the photo for transducer recognition:
Does anyone know if the changes were just physical or were there electronic changes as well??