The picture is of a receptacle that can accept both 15 and 20 amp connectors. A 20 amp will have one 'blade' rotated 90 degrees.

I'm pretty sure the 770 uses a 15 amp cord - if not it would be one of the first pieces of consumer equipment I've ever seem with a 20 amp plug. Also, note that the 770 is rated to draw at most 1800 watts, or 15 amps .....

Where did you see any indication of a 20 amp plug on the 770?