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It looks like it's mostly rust. I would use a wire brush with a rust penetrant to loosen it. Once you have the rust cleaned as well as possible, it may still take light sanding with 400 and water as a lubricant to reveal the stamping. You just don't want to use anything aggressive for sanding.
The stampings for the trans should be on top. It will have a coded assembly date and should have the last 6 numbers of the vin too. They can be almost anywhere, but the top was the most common, usually on the main case, but sometimes on the midplate.