There is no "A2" buffer tube. There is only the "A1" rifle buffer tube and by adding the small extension adaptor, you can then install an A2-length stock on the rifle buffer tube.
Like wart mentioned, the rifle buffer tubes do not use the retaining nut.
The carbine buffer tube is shorter than the rifle buffer tube. There is no A1 or A2 for the carbine buffer tubes, only commercial or Mil spec carbine buffer tubes.
Since you have an A2 stock on your DPMS, it's safe to say you have a rifle buffer tube, not a carbine buffer tube.