Forced Air Register Question

I have a house that was built in the 1960s with forced air. The registers (both supply & return are in the baseboards.) In the middle of the registers (typically 4' long along the baseboards) is a large screw.
At first I thought that the screw held everything together, but then I noticed the returns do not have one. I was able to turn the screw (its edges are serated like a quarter's to make it easier to hand turn) with a large screwdriver (and some effort) and it is definately connected to some sort of baffle inside that runs the length of the register. Unfortunately turning the screw doesn't seem to do much, either to the baffle or the air flow. The screw comes out a little, but that's it.
I would love to be able to balance my system better (less air out of the kitchen register which is over the furnace and open up another register). There are no butterflies in the ducts or any other way to adjust air flow.
Is anyone familiar with this type of setup? How are the registers adjusted? I want to see if something inside isn't working properly or if I just don't know what to expect. Taking the registers apart wouldn't be easy and they have been painted at least once.
Unfortunately, they aren't like floor registers that you can easily remove if the mechanism is stiff. I'd appreciate any advice!
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I'm afraid, far's I know, removal is the only way you're going to be able to gtet at them. If you don'th ave the butterflies in the ductwork near the furnace, then all you have left to balance with is those screw adjustements. And yes, you're right; they're finger adjustments that rotate/set "fins" inside to block air flow, very similar to a regular floor register. Grab a flashlight and see if you can get some liquid wrench or similar stuff onto the working parts; might loosen it up enough to work some.
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