As far as I know,  Underwood 5's came from the factory with white, glass-covered keys, though I am unsure of whether they came in white the last couple years of production.  When you see key-top variation on No. 5's, it is because the key legends were replaced at some point in it's history, which was a common procedure during servicing, and many of the examples you find today have key-tops that are different from the factory originals, either in color or material, because they were replaced back in the day.  Even if the keys are white legends under glass, they may be replacements, as this was pretty routine.

The Underwood 5's were produced for over three decades, so there are some slight variations in the design depending on the era.  There were also sub-models with different feature sets.  Along side the Underwood 5, the Underwood 3 and 4 were also sold, the 3 being the wide-carriage standard offering, and the 4 having less characters (76 vs. 84).

If you go to the Typewriter Database, you can find numerous examples of Underwood 5's from the earliest to latest. Some easier to spot visual differences in eras were the shift lock changing from a ball to a key in 1915, and the frame redesign in 1922.