I've seen that problem on x01 series before. 631 calls out a 601 series. The D means diesel. I've only seen one tractor with an S after the D and it had a Sherman aux tranny. I assume the S stands for Sherman. Does yours have a sherman? If so it will have a shifter handle on the left side of the trans case. the serial number should be under the 631 DS. May have succumbed to battery acid too. Check the site below for better info. Good Luck.