Boat has one owner and the original graphs but the other day noticed what appears to be a evidence of a transducer mounted. But has an in hull transducer? Any ideas?

Would a Side scan/down scan ducer work here?

Temp sensor?

My guess would be a speedometer. All of the transducer mounts I have ever seen were of a square or rectangular shape with slots to allow up and down adjustment.
A side imaging transducer is going to shoot out a thin, horizontal beam on each side from straight out to under the boat. Anything that is in line with that beam will interfere with the signal uncluding a lower unit of a motor..

Looks like this might match up, speed sensor.

Ah, I guess because the Johnson has a built in speedometer so they took it off for the dash mounted.

I'm hoping I can use the standard Lowrance mounting block to match the holes.