I looks like "D" serial numbers are 1995 if you look on this website - http://www.tractordata.com/farm-tractors/000/7/5/754-massey-ferguson-231.html
1968 http://www.masseyfergusontractorparts.net/Massey-Ferguson-tractors/2008/06/massey-ferguson-135-serial-numbers/
1968 http://www.masseyfergusontractorparts.net/Massey-Ferguson-tractors/2008/06/massey-ferguson-135-serial-numbers/
model 880116 1961
mf35
1958
You have a 1954 model TE-20.
That is at least a 1975 tractor. What model is it?
1972
It is a 1959.
007311
1981
I believe that will be a 2000 model.