Essman,
Regarding your last 2 note (4 & 5), It sounds reasonable that the original depth sounder transducer was mounted in the location of the 2" capped through hull. Their was abandoned wiring in that location and another transducer mounted nearby. I replaced the latter with a new device.
Thanks for pointing me to an other section of the forum. I'm still picking my way through the various sections when I have time.