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You have likely purchased a 641 with a replacement transmission from a 1962' 2000 tractor OR the tractor is actually an industrial low-center of gravity (LCG) 2000 with 641 "tin" on it. Here is a link
to a good site that has some info up through the 4 cylinder thousand series.
If you post a pic, it will be easy for others here to let you know which you have, the ag 641 w/ replacement tranny or a 62' LCG 2000.