That is the top thread tension. I can't see exactly how that one is made, but center it so the ends on the pedistals and then you run the thread from the spool on the right to the tensioner. There should be two pieces to the tensioner spring ( the large flat part held by the screw) and the thread passes between it. As you turn the screw in it tightens the tension, backing it out reduces it.
My New Home has that style. Works good, but is not controlled by the presser foot lifter as in other type.
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Also if you soak your attachments and the unpainted pieces in Evapo-Rust it will clean them up and they'll look better.
Joe