Springfield Trapdoor Buffington sight question

[KnotRight]

I got a Trapdoor about a month ago and love it. After I figured out the gun was sighted in at around 250 yards. It has a Buffington sight on it and not sure what the triangle cutout on the sight. I know that the lower peep hole is used for the range on the right hand side of the sight and the top peep hole is used for the left hand side. But what is the triangle cut out used for?

[M-Tecs]

http://www.trapdoorcollector.com/cgi-bin/afrasca/bulletin/config.pl?read=34505

"The sight was designed by the same man who designed the 1901 Krag rear sight and the 1905 rear sight on the 1903 Springfield. They are Excellent sights that when properly used, can enable the shooter to place shots quite accurately. These sights are perfect for target shooting. Take some time and study them. Use logic in their use and application. Their use is actually quite simple and obvious when you look at the graduations on any model of Buffington's sights. No math need be involved. The trapdoor Buffington sight has been calibrated with regard to the load of the carbine or rifle round for that model. Barrel length, muzzle velocity, trajectory, drift, how much that grain weight of bullet will resist the pressure of the atmosphere in its travel, how much it will slow down from air resistance and how temperatures and altitudes this occurs, etc. etc. The sight is made simple with lines running from the peep or triangle (Field View) openings of the rear sight to whatever distances in hundreds of yards that the shooter wishes to adjust the sight to. You'll be able to figure it out. "


http://www.trapdoorcollector.com/shooterstext.html

[KnotRight]

Thanks M-Tecs