"mambru se fue a la guerra," as I also found out when I was looking for information, has a multiplicity of hits, but few are useful, and many are Latin American rather than Spanish. A number of them are anti-war or politically-biased and most have no real information on the song and many just copy of each other.

A Spanish website with good lyrics is: http://www.terra.es/personal2/anarco/cancion/mambru.htm. This one has the pio-pio verse.

A children's site I used when looking for versions of Don Gato also has the pio-pio verse: http://personal.iddeo.es/bernal/marisa/cuentos/canciones.htm

A more useful site is: http://fresno.cnice.mecd.es/~gmat0002.didact/actividades/partituras.pfd/mambru.pfd- which has a version collected in Segovia, a different last verse, and sheet music.