According to this Slim Dusty discography, which covers only currently available albums, ONCE WHEN I WAS MUSTERING and SADDLE BOY are both on the album "Songs of the Saddle" (first released July 1963)– which makes it very unlikely that they are the same song.

Lyrics to SADDLE BOY are available at several places on the Internet. In the usual sloppy way of most lyrics web sites, they attribute SADDLE BOY to Kasey Chambers, who recorded it, but didn't write it.