Actually, IYT has markers six lines out on their 19 x 19 boards and only five lines from center on their 13 x 13 boards. Neither of these correspond to P1's move #2 restriction. Pente.net's markers are six and three lines from center on their boards and you can play with or without the restriction on either size board. BrainKing has no markers at all. I believe the way pente.net has their markers on the 19 x 19 board is the way they were originally done by inventor of Pente and by the various game board manufacturers through the years, but I'm not certain of that. Only DSG has the markers 3 lines out.
So now we've tripped over something interesting. Why are the markers traditionally placed where they are (and where exactly do they belong)? Anyone?