There are indeed several processes used by law enforcement for viewing milled over/previously stamped serial numbers. Both principles work to provide an image of the stressed metal area that has a more densely concentrated molecular signature (the stamped numbers) than the rest of the engine pad (the unstamped area). One involves heat, which is a one-time-only process that requires photos to be taken of the area in a very time sensitive manner as the image fades rapidly and is gone forever as the molecules in the metal move back to their prestressed (prior to stamped) condition. The other method involves chemicals that can make the prvious stamping visible to the naked eye. Consult with your local forensic evidence person for more details. If you have a large county sheriff or prosecutor's office in the area you may want to broach the subject (no pun intended) with them. Many times they are actively looking for items of non-evidentiary value that they can practice their procedures on without risking blowing a criminal case if the process doesn't provide the desired result.
Sincerely,
Cliff Clavin, resident expert on useless trivia.
