Hardened steel punches with number or letter characters forged into them.
How to use a Stamp Set:
A good set will have numbers 0-9 and the alphabet A-Z. When hammered on, they will leave an imprint of the character in the part to be labeled.
Stamp Set as seen in Basic Engine Building:
Parts like connecting rods, rod caps and even the timing gears are excellent candidates to be stamped. You don't want to put a rod cap on the wrong connecting rod.