I purchased a used FSU from an ebay seller and seems to work fairly well off and on (first couple times I tested it out it didn't register anything on the gas gauge, but currently it does). It's about a mark or so from the empty line on the gauge when the lever is completely down, and quite a ways past full when completely up.

I took the metal cover off with the 3 tabs and carefully removed the very light rust on the tiny metal tab inside and looks clean now.

I know there is a way of calibrating the units with the torx screws, but as you can see this one is different. Any way I can calibrate this one? I'd rather not have to bend the float rod.

There isn't a way to calibrate. If the wiper and the wire winding are making good contact it should be around 0 ohms empty and 90 ohms full. The problem can also be in the gauge on the dash.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

Or iffy wiring to them or almost anywhere in the car.

When using a meter on the sender itself, w/ or w/o a Rel button to Zero a digital meter...
Empty 0 - < 1Ω
Full 90 - 95Ω

And when you move the level between the 2... often value changes some every time.

That's even with OE sender.
Car Maker never made them to be very accurate. Even then Many floats are set read "empty" before the pumps are sucking air to prevent pump failure. All Electric Fuel Pumps need gas flow or quickly dies from overheating and no lube.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave