missing serial number, yard machine 21 hp gt

i have a yard machine, 21 hp, twin 2, gt. the serial number is missing, it was given to me after it sat 2 years in the snow. it has a 46 inch deck on it, and the engine model is 461777, type is 0137, and the code is 9809035b. i know it is at least 5 years old. right now i have it torn down so i can rebuild it, but i don't know the year or model. can u help me out

missing serial number, yard machine 21 hp gt