10 series tractors were generally built from 1983 to 1986. Manual or power-assist steering, less hydraulic capacity than later models. 15 series (1215,1715,3415) generally from '93 to '97. Stripped down economy models. 20 series built from 1987 to 1999 full line of compacts most with hydrostatic power steering, synchronized or hydrostatic transmissions (hydros on smaller units only), live PTO. Very durable and popular. Today's TC's are largely based on 20 series drivetrains. Hope this helps.