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When I print my scorecard, I am noticing small purple letters in some but not all frames below and to the right of homeplate on the graphic (e.g. F, G, L or P). What do these signify?

They stand for "Fly ball", "Ground ball", "Line drive", and "Popup". It is hard to draw an "arc" like we do on the scoring screen for fly balls, so we just designate the type of hit ball with the letter.