If a 54 indeed has the straight type like my 53, that is B&G part number 184-00-076/FI Bought a pair( replaced & working) in August 2019 here: https://outletspecialist.com/catalog/product/view/id/1415/?___store=nl
Just found a new offer here: https://maxmarineelectronics.com/product/b-g-bg-sonic-speed-replacement-transducer-tube-lg-bung-sensor-184-00-076-f/
Hope this helps you,
Stefan Jeukendrup sv Malaka Queen Super Maramu 2000 #348
