Can anyone tell me the difference between the Square and round top Steering box? I have a square top and need to replace it, it's shot and I have the opportunity to replace it with a new round top at a very reasonable price.

For the 81, there were two sizes. The small gear (605 model) and the Large gear (800-808 model) THe small one has a round side cover, the large one has a rectangular side cover held in place with four bolts.