The engine you are looking at is not a 20hp.
It will be a 25 - 30 - or 35hp engine from the years 1955 to 1958.
The model tag is not on the transom clamp on this engine either. It is located on the front of the engine below the steering bar, where the tilt release is located.
They are great engines if you are mechanically inclined. Once properly brought back into service they will run until you hand them down to the next generation.