"M" is just a series of prop (for lack of better words, or correct words).
It's a letter that Yamaha uses to designate a particular style of propeller.
Example: 23M, 25M, 27M, etc. (those would all be YAMAHA manufactured propellers)

I like the "M" series props. Good all round useage three blade prop for a bass boat.
A 25M would probably be a good choice for your boat.
Does it not have one on it now?