That was a fun detective mission - the names I came across tracking that down! Roth. Fink. Ames. FerroLuzzi-Ames. I am a scientific grand-child of Bruce Ames, and my graduate work was somewhat in competition with FerroLuzzi-Ames, so this was pretty neat.
To answer the question, according to Roth, Anton and Hartman (Table 1 in J. Mol. Biol. 22, 305-323, 1966), hisO1242 was a spontaneous mutant. So it most definitely could occur naturally.