Casting codes can help you determine when the parts were cast at the foundry, but not when the tractor was assembled. Engine serial number can get you CLOSE, but not real close, to the original serial number.
Basically, you're out of luck finding the ORIGINAL serial number, unless you can trace the tractor back to its original owner, and/or the dealer it came from. If the original owner isn't long since passed on, and/or the dealer long since gone out of business, you have a slim chance that they kept a copy of the original bill of sale for your tractor. On that bill of sale will be the original serial number.