Mosler Safe Co, in their US patent, 3111022, covering Tamper-Proof Locks, states that a "rather frequently"used technique for surreptitiously opening a combination lock was to heat same moderately, without even melting or permanently damaging same, when a sufficiently skilled person could open the lock without knowing the combination.

I had never heard of this method, and neither have others in the trade! As this was in the sixties, were thermal auxillary locks introduced in a rush? Was the heat altering clearances, as no permanent damage occurred?

One hears so much about locks proof against Radiographing, but presumably this would only be of use for small, thin walled safes, where back-scattering methods (Even neutron-absorption techniques) might be used. Surely no vault door or other thick steel door was susceptible. Perhaps it was used where a door was left open and unobserved?