The packing number of the double vertex graph of the path graph

Neil Sloane showed that the problem of determining the maximum size of a binary code of constant weight 2 that can correct a single adjacent transposition is equivalent to finding the packing number of a certain graph. In this paper we solve this open problem by finding the packing number of the double vertex graph of the path graph. This double vertex graph is isomorphic to Sloane's graph. Our solution implies a conjecture of Rob Pratt about the ordinary generating function of sequence A085680.