ALL older Catalinas were manufactured with 304 SS tube to make the Rails and Stanchion tubes. 304 is stronger than 316L but less corrosion resistant. Somewhere you might have a non stainless bolt or screw causing that mess, but also like others have mentioned the tubes can actually rust on the inside.
I wouldn't be surprised if you dismount all that stuff and broken off screw or bolt comes tumbling out the tube, all rusty. You are in Canada, so this looks like a good winter project for the cold months. You'll want to take everything off (its a royal pain that takes two people). Like Skipper said, replace all the screws. Order from a reputable hardware place online so the you know you are getting 316L alloy machine screws & nuts.
There are various chemical cleaners that deal with rust. CLR is one, but not very strong (it used to be stronger). If you know a welder or you live near a welding supply shop try to find a metal cleaner/descaler/degreaser that contains phosphoric acid as the primary ingredient. Don't worry, its just a spray cleaner and won't burn you, just don't get in your eyes or nuthin (same safety precautions as many household cleaners). The stanchions can be soaked in a bucket (you can cut the cleaner with 2/3 hot water). This will clean out all the rust from the inside of the tubes. For the bow rail, you'll have to just spray the inside of the tubes. It would hard to have a vessel deep enough to submerge the whole thing into a solution.
When all that stuff is off your boat it would be a great time to epoxy 'pot' all the mounting holes. This is also a great way to leave them sealed up for the winter while you work on the other stuff. Over-drill all the holes to 5/16", tape them off below and fill the holes with epoxy. Leave them sealed up for the winter, in the spring drill them to 1/4", that leaves you a nice seal of epoxy around the bolts so if water ever gets in it won't harm the wood core.