Not much to add to Scotts comments.....but I'll give it a go.
The Suburban 400 was only made one year....although most Horses up to about 1968 were considered one year models with minor changes & improvments, but a new model number every year.
They were powered by a 4HP Kohler, had the 3 peice tranmission with 3 forward gears/one reverse & were very capeable despite only haveing 4HP!
The 1960 models had many changes & improvements over the 59 & earlier Horses going back to the original Ponds.
In 61 Wheel Horse offered the 401/551 models that had subtle changes over the previous years 400/550 models.
61 was also the last of the mid engine "ball burners" Horses.
The Suburban 400 has two levers below the steering wheel shaft for the throttle-choke controls.
They are unique to the model & year of the tractor....don't lose them or force them if there rusted.
It would be easier to find a vegitarian alligator than another set of T/C levers.
The 400's are a cult classic among many GT collectors & are very sought after.