I had a very hard time finding the numbers on my '32 frame. Having a background in Materials Engineering I knew that acid etching could help and I was able to get the numbers using HCL acid.
"Acid etching is a forensic technique used to recover obliterated serial numbers by exploiting the fact that stamping compresses and hardens the metal grains beneath the mark, causing the deformed areas to etch at a different rate than the surrounding undamaged material. The process typically begins with polishing the surface to a mirror finish to remove superficial scratches, followed by the application of specific chemical reagents that create a diffuse reflection in the stamped areas, making the hidden number visible.
Common reagents are selected based on the metal type:
Steel and Iron: Fry’s reagent (ferric chloride, concentrated HCl, and distilled water) or Nital (nitric acid and ethyl alcohol) are frequently used to reveal marks erased by filing or grinding. "