Jim,
This "marker" title flummoxed me, too. However, I have an educated guess and since nobody else came forward with an answer...
Jim, my guess is that the commander of that lad's platoon, company, or whatever noticed that the man in question was always reliable to get in place quickly when the command was given to "fall in," as if into formation. To have a man you can count on to do that quickly each time you need it done in a hurry is a great asset to a small unit. Sometimes in a battle zone the seconds count to get set up so you can get going before artillery sees your formation as a target. You see, from the standard unit formation you can peel off in column of two's or column of four's or anything the situation or the terrain dictates. However, when the men are tired or upset, sometimes they don't "hop to" as quickly as you would want and their tardiness could mean trouble or danger. My guess is that the "marker" was the man designated to the front right corner or cornerstone position upon which the rest of the platoon, company, or whatever falls in line with. I could be wrong, but that is my guess. I don't think "marker" is the correct name for this corner man's position according to the book, but I can see how somebody could use that as a local term.

Bruce Nichols