I guess the wall is divided into two main sections -- everything up to 1969 on one side and everything up to 1975 on the other? I seem to remember seeing those two years posted at the right-hand side of each section.
My Canadian friend wondered why I was so subdued while we were there -- he asked if I knew anyone whose name was on the wall.
I told him I was subdued because as an American with a bit of historical perspective I was too cynical to spend much time there in the right frame of mind. In my opinion, a more appropriate Vietnam War "memorial" would have had statues of Lyndon Johnson and Robert McNamara in a hollow pit in the ground -- and visitors would be invited on a daily basis to bury them in crap up to their armpits.