The 850 and 950 were the first compact utility tractors introduced by Deere. I have some brochures from both 1977 and 1978 that feature the 850 & 950, but neither were initially available with mechanical front wheel drive (MFD). That option must have come a couple of years later. FordTruckGuy is correct in that these initial compacts were a partnership with Yanmar and Deere.
I have a Grounds Care Purchasing Guide from 1983 which includes the greatly expanded compact tractor line. The line now includes the 650, 750, 850, 950, 1050, & 1250. All of these tractors were available with MFD.
The research that I did indicates that most of these additional models were introduced around 1980 or 1981.
I hope that this answers your questions.