Hi namdnas, I heard of a fellow who went to the police to find out if the motor in his L34 Torana was original as the number had been ground off in a rebuild and then restamped ( but incorrect lettering size ). All done by a previous owner of the car.
Anyway, the police used some chemicals and managed to show the original number stamped in the block was the same as the number stamped. Something about compressions in the cast blocked from being stamped showed up darker than the rest of the block. Hope this makes sense. Go to the GMH Torana forum and you may find the article there somewhere.
I would imagine you would have to know a policeman fairly well for this kind of help!