Garmin has changed the power cable pin configuration used between different generations of Fishfinder products.
Newer products like the echo series use a 4 pin power cable. These devices use a 4 pin transducer.
The Fishfinder 300c and 400c use a 19 pin power cable. These devices use a 6 pin transducer.
Any older (discontinued) Fishfinder uses an 18 pin power cable. These devices use a 6 pin transducer.
As long as the pin configuration is correct with the newer device, the power cable can be used.
The transducer being used may need an adapter to work with a newer device.