METHODS COMMONLY USED FOR MARKING OBJECTS WITH IDENTIFICATION NUMBERS ARE (1) BY INDENTING A SERIES OF NUMBERS INTO A SURFACE USING A HARDENED STEEL PUNCH, THE FACE OF WHICH BEARS A MIRROR IMAGE OF THE NUMBER; (2) BY PUNCHING SYMBOLS INTO THE REVERSE FACE OF A THIN SHEET OF METAL SO THEY STAND OUT ON THE FRONT OF THE SURFACE; (3) BY INCISING SHALLOW GROOVES INTO A SURFACE WITH A SHARP TOOL; AND (4) BY 'WRITING' SYMBOLS ON SURFACES WITH WELD METAL DEPOSITS. AN EXAMINATION OF THE MICROSTRUCTURE OF A METALLIC COMPONENT CAN REVEAL A GREAT DEAL ABOUT THE COMPOSITION OF THE METAL OR ALLOY, ALONG WITH MECHANICAL AND THERMAL HISTORY OF THE COMPONENT. THE CHEMICAL TREATMENTS WHICH ARE USED IN THE RESTORATION OF ERASED NUMBERS REVEAL ONLY THE COARSER DETAILS OF THE GRAIN STRUCTURE (THE MACROSTRUCTURE), BUT THE MICROSTRUCTURAL CHANGES PRODUCED WHEN IDENTIFICATION MARKS ARE PLACED ON ITEMS UNDERLIE THE EFFECTS THAT CAN BE OBSERVED. WHEN A NUMBER IS INDENTED INTO THE SURFACE OF A COMPONENT WITH A PUNCH, THE GRAINS AROUND AND BENEATH THE INDENTATION ARE DEFORMED. THIS DEFORMATION IS KNOWN AS 'COLD WORK.' THE THIEF OFTEN REMOVES JUST ENOUGH MATERIAL FROM THE COLD WORK TO DEFACE THE INDENTATION. WHEN SUCH AREAS ARE ETCHED, THE COLD WORKED REGIONS DISSOLVE MORE QUICKLY THAN THE SURROUNDING AREAS, AND FAINT OUTLINES OF THE ERASED NUMBERS CAN BE SEEN. THIEVES NORMALLY REMOVE WELDED-ON MARKINGS BY USING PORTABLE GRINDERS, LEAVING ONLY ROUGH SURFACE. HOWEVER, EVEN IF ALL TRACES OF THE WELD METAL DEPOSIT ARE GROUND OFF, IT IS NESESSARY TO GOUGE DEEP BELOW THE SURFACE TO REMOVE ALL OF THE HEAT-AFFECTED ZONE. TREATING THE SURFACE WITH DILUTE ACID REVEALS THE ORIGINAL MARKINGS. THE SMALL AND SHALLOW INCISED AND VIBRO-PAN MARKINGS PRODUCE LITTLE COLD WORKING OF THE MICROSTRUCTURE UNDERLYING THE MARKINGS; AS A RESULT, ERASURE OF SUCH MARKINGS IS USUALLY EFFECTIVE, AND RESTORATION IS OFTEN DIFFICULT. CHEMICAL TREATMENT OF THE AFFECTED AREA MAY, HOWEVER, REVEAL THE IDENTIFYING MARK. PHOTOGRAPHS ILLUSTRATING ACTUAL CASES OF RESTORATION OF IDENTIFYING MARKS ARE PROVIDED. (RCB)