Kubota does not, nor any tractor mfg, produce tractors by model year. Best you can do is have a dealer run the serial number to find out when it was first sold.
I can't remember back that far but is or was their a sticker/decal on the engine stating it meets EPA codes for a given year? My BX has this on the engine and I use that as a reference for it was probably with-in a year or so of that date.
Is this a valid thought or just another bad idea from a senile citizen?