Oliver probably built several hundred models of walking plows in their one hundred thirty + years of producing plows.
The model 40 walking plow was probably the most popular. The model 20 you are asking about was a slightly smaller version of the model 40.
Your plow would have been built sometime after that patent date. The "SB" letters should stand for "steel beam". The plow was painted red when it left the factory. I hope this helps.