In order to keep the tractor from freewheeling downhill at any speed there was a separate add-on holding valve that could be added but starting in the late 70s it became an option from the dealer. In the mid 80's they began using a travel control valve that had the holding valve built into the travel control valve.

The original tractors had the 'elephant ear' fenders.  As time went on they became more full, and eventually complete fenders over a few different style changes.  The newest models have full fenders that will meet the floorboard of the tractor making a full operator's platform.

With the 3000/4000/6000 series, the engine flywheel was spun 180 degrees. Before engine air was blown on the owner, many did not mind it in the winter as it kept the operator warm especially if a cab was installed. But In the summer it could get hot

In 1980 with the addition of the 448 they added 2 inches to the frame of the big wheel series making it a 'long' frame.  It was done so that the 60-inch deck could be mounted under the tractor - it also does make it a bit nicer to get on and off too!

The Latest models will have the "webbed and gusseted" rear end that was previously only used on the Backhoe Loaders because of its added strength and weight. In the early 2000's it was also used on all 3000/4000/6000/7000 series models. It was stronger but more likely much easier and overall cheaper to warehouse a single-style rear for production and cost advantages.    

With the 3000/4000/6000 series, an electric PTO for belt drive was used instead of the previously used manual engaged style. Because of engine position being spun now changing belt direction rotation, the mower deck discharge was now on the right side. Now the Hydraulic PTO valve option was located under the seat instead of on the fender.  Power Steering was also now an option!