Gary, you are quite correct on the originals being affected. I"ve heard the 66 IH tractors from the 1970s are really falling off too. Not surprising to me though with the versions done in the 1990s and more recently again.
There are a few tractors where just too many have been made, like the John Deere A, Farmall M & Oliver 770. All are considered classics by most but people will only want to buy so many at a certain price.
Just about any Allis-Chalmers toys are doing quite well, the best prices I"ve seen, though I"ve only beeen in it seriously for 9 years.