if you can remove it, put it in an electrolysis bath.
I am also restoring a 1944 John Deere B tractor and could not decipher all the numbers on the SN. Could get enough to get the year, but that was all.
Take the plate and place in a plastic container. I have found that plastic Maxwell House coffee cans work best (have a recess for the handle that seperates the electrode from the part.
Fill with water and about a teaspoon of baking soda, wire some baling wire or some other wire through the hole. place in the water. take a piece of salvage metal and slip down in the hole. take a old fashioned (non-automatic) battery charger and connect the positive lead to the salvage piece and place the negative to the part needed to remove the rust/oxidation. turn the charger on and let it sit for a few hours. remove. it will oxidize a little, but will wipe off. should make it more readable.
On a related note, this works on any metal, to remove rust. save the busted knuckles and worn arms. you can put as big a part as you can submerge in a non-metallic container. You can also add some lye to the water and not only remove rust, but paint as well. Has saved me HOURS of work.
