It's called peening and was one acceptable method of stopping a screw backing out of a nut in those days. Another that I noted somewhere in the programme was the use of several equally spaced centre-punch 'pops' at the emergence of the threads of the bolt from the nut. The 'pops' distorted the thread of the bolt and the surface of the nut, blending one into the other and preventing rotation.
I've had a quick web look but can't find a reference for these two methods.