It has been decades since I've had the bottom end of a Gravely engine apart. It sounds like the "plug with a screwdriver slot" you are describing is the head of the spreader bolt. I'm just guessing that you would "tap" the other end of the bolt to knock the entire spreader bolt out of the crank pin.
You don't mention the vintage of your "L". They were made from the late 1930's through the early 1970's. I don't think that the bottom ends changed much over the years, but the part numbers did change. You may want to look at some of the Gravely manuals that area available free online on several web pages.