Thanks--just to be clear, I've already installed a new system (not a great system! Striker 4, but it does the job) so I'm not trying to use this as my primary fishfinder.
That said, I love old tech (bit of a tinkerer, or maybe a hoarder) and I would love to figure out of this current system works. I understand that most people would toss it out, but this is exactly the kind of old gear that I would rig up to my nephew's little jon boat or something so he can at least have a depth gauge

rather than throwing it away!
So here's my thought process: the cables go deep into the helm and get tangled up with a ton of other cables. I have no idea if there's a transducer buried somewhere in my hull or not. For me, the time it would take to trace the wires, get down into the hull, figure out what's what, etc. is not worth it if this system doesn't work. I would much rather spend the, what, $100 on a head unit and plug it in and see if it works. If it does, great! It's probably worth my time to dig the system out and then I can do something useful with it. If not, I'm really, really glad I didn't spend all this time tracing down wires for a nonfunctional system. And oh well, I bought a head unit that now I need to resell on ebay.
So, I really do have a specific goal and thought process, and while I appreciate the advice that I should just replace it, that's good advice for most people but does not apply to my situation. I already have a newer system, I am specifically wanting to track down the unit that works with this cable and test this specific old system. I might cost myself some money to rescue an old system, but I'm okay with that
I get how it would be hard to do this without knowing the exact pinouts, etc. But, I figured I would post this here on the off chance someone has this same system and can say "oh! That looks just like my [whatever], same pin pattern and connector" and then for me it's worth the money to get an old one and plug it in. But if not, all good!
Thanks all!