Water in the cylinder

It does look pretty clean. When coolant enters a cylinder of a running engine the heat vaporizes, literally steam cleaning everything inside the cylinder.
It it is also possible that the Previous Owner removed the head for some reason (& drained the coolant for that) but had to stop working on it for some reason so after cleaning things up he and put the head back on without tightening the bolts fully to keep it clean inside (the carbs & exhaust would have had to come off for that too).

Welcome to the forum. Please add your location and your bike's model and model year (NOT year first registered as UK paperwork shows) to your profile so that you don't have to remember to tell us every time and we don't have to keep asking when you forget (see Forum Settings link in my signature).

And welcome to the world of antique vehicle ownership (they own us, not the other way around). Your bike is about 4 decades old and may or may not have had all of the maintenance necessary to keep it safe & reliable 9even before it deteriorated for so many years) so it is highly recommended to download the Factory Shop Manual for your model (available through the CX Wiki - link in my signature) and go through all of the service procedures, regardless of whether your bike has reached the specified mileage.
Note that while aftermarket shop manuals are pretty much necessary for people without factory training to work on a lot of makes & models of bike the FSMs for the CX/GL500/650 family of bikes are so well written & laid out that the FSM is really the only book you need and and the aftermarket books are secondary references at best.

I also recommend looking on all rubber parts with suspicion because rubber does not age gracefully. If it sat that long you don't need to check the date codes on your tires to know they are over 5 years old and should be replaced no matter how good they look & feel (old rubber simply cannot flow around the irregularities in the asphalt well enough to grip, especially if it is cool or wet).
The original rubber brake lines should be replaced every 2 or 3 fluid changes (= 5 or 6 years) so if your bike still has them I recommend shopping for modern stainless braided ones (they last practically forever and double the life of the fluid).
And don't forget things like the rad hoses and the boot between the engine and swingarm (they can crack on the bottom where you don't see it).