Matrix Games Forums

I know I asked this a long time ago but I can't find the post. I'm redoing some of my parks using the old photos that were recently shared, and can't remember what some of the ball flight locations mean. What does the M and/or X mean? I'm thinking the X means Extra but am not quite sure what the M means or how it relates to the other positions.

Here is the key: http://www.retrosheet.org/location.htm

Thank you for that excellent source, good sir!