Have owned a few Humminbird FF, mostly from the early 2000s, just got the 698si. All have used the square transducer cable that fits into the quick-release mount. Digging through my box of parts I found a transducer tagged SHS-7W that has a round 7-pin connector with locking collar. It appears to match pics I can find of that model. These transducers were used on a bunch of the 3D models from years back, and this one is surely at least 10 years old. That I can tell, all of those models similarly use the quick-release mount, meaning this cable wouldn't mate up.

Anyone have any idea what model of FF this was meant to fit?