NA connects numbers in an additive fashion:

              +--NA-+--ND-+--AN-+ 
              |     |     |     |
          a fifty five gallon drum . 

It is also used for spelled-out dates and times:

            +------Jp-----+
            |     +---NA--+
            |     |       |
          in nineteen twelve 

The attachment point between the number and the rest of the sentence
is at the least-significant place, in the same way that "NN" 
links attach (for example, "I gave him two hundred").

NA will also connect "and" when used in numerical expressions:

            +------------Op------------+         
      +--Sp-+     +--NN-+---NA--+--NA--+-Mp-+-J-+
      |     |     |     |       |      |    |   |
     We bought.v two hundred and.sum three of them