What would cause a 120 VAC universal motor to run slower in reverse?
Your motor is 'timed' to operate in one direction by advancing the brushes. This is clearly visible in picture #2, where the brushes look to be rotated anticlockwise by about 12º relative to the stator.

Advanced timing increases rpm in one direction, but reduces it in the other direction. A motor that is 'neutral' timed will go the same speed in both directions.  

The timing on your motor does not appear to be adjustable. You could try removing the brush holders and springs from the board and drilling new holes for them, but it may not be worth the effort.