use of X-ray
I used the services of an industrial non-destructive testing facility. They use x-ray to check metal objects for cracks, joints etc. I trust such facilities are available everywhere. Try "non destructive X ray" on Google and you will get thousands of contacts. It is not too expensive. A whole series of exposures (and there were many) cost me less than 200 dollars.
Regarding revealing of a stamp or signature, it depends on the thickness and depth of the stamping. X-ray does not exhibit the best spatial resolution because of what is known as secondary emmited radiation (compton effect).
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