The V shown on the back side. Notice the 1 to represent the year 2001. The next photo shows the base with an 11 on it. That represents the year 2011. These were part of the original block that was sculpted. They were bore holes made with huge equipment as they were cut out of the original slurry wall of the WTC. There are no accidents in art, San always says. She took what some might of viewed as an imperfection in the stone and made it part of the sculptures design instead.