How fast was Walter Johnson's fastball?

Sports historians who have seen Johnson piotch on old newsreels have estimated that he ONCE threw a fastball close to 120 mph. I doubt that. Unfortunately, there were no radar guns in those days, but a munitions factory once calculated his fastball at 92 mph, a speed virtually unknown in his day, except possibly for Smoky Joe Wood. In any case, he was the fastest pitcher of his day, and his speed is all the more awesome because he was a sidearm pitcher, as opposed to most power pitchers who come straight overhand.