Hmmm. Looks more like an SS than my Master, but that is not to say it is not a Master. Look for Mxxxxxx serial number. The Master also has a removable panel that is part of the front of the typewriter. On the SS the entire front panel is removable. The Champion keyboards from what I know were optional on many of Underwood typewriters. Champion keys are not glass top keys. They are larger than the glass key tops. Underwood made so many variations of their office machines it can be difficult finding all the variations. If it has the M in the serial number it is a Master. I could see what looks like Champion on the space bar rear bevel. In any case it should be a fine typer.
I forgot about the Underwood S. It looks like an SS and has regular glass key tops. If I recall the regular glass key tops are about .400″ measured across the O.D. of the ring and Champion key tops are .500″ measured across the O.D. of the rings.